With Vos and van Vleuten, there is still an element of, for many years the sport's pinnacle was a self-fulfilling prophecy. Pre-WWT days and while most of the major races were organised by enthusiastic amateurs while people like ASO refused to broadcast a single second of the few races they bothered to arrange, the lack of money in women's cycling often meant that only the small handful at the very top of the sport were able to dedicate themselves full-time, and as a result of that they would typically then dominate because they were beating part-timers, and so the rapid development of the WWT has meant that some of the veterans of that era were still getting that benefit late on in their careers. Vos was crazy successful very young, which meant that her longevity also makes her seem older than she is, after all she's five years younger than van Vleuten but was already an established champion when van Vleuten went full time already in her late 20s.
The other thing was that with the races largely organised by enthusiastic amateurs on shoestring budgets, parcours trends became rather samey, even further playing into the hands of the select few who could maximise their results by giving them hegemony over the largest part of the calendar. Races frequently focused on sprints and/or hilly circuits where willing hosts would see multiple stages around a small geographical area, often starting and finishing in the same town or being a pure circuit race, or simply padding distance in point-to-point races by adding flat loops of the start town or similar. This meant that a certain type of rider was explicitly favoured by the parcours trends of the time, and the péloton did not get the chance to separate into individual types of specialist; a one-dimensional TT rider or mountain goat was simply not valuable in enough races unless they were a complete outlier, talent-wise, like Mara Abbott. This actually got worse, not better, in the 2000s; as Fabiana Luperini aged out and a few doping scandals rocked the Giro Donne, the parcours simplified (seeing riders like Edita Pučinskaite and Nicole Cooke complaining about the race being made easier) and race days reduced, while the 2008 financial crisis put paid to the ailing Grande Boucle Féminine and the Tour de l'Aude, taking two previously challenging stage races that had included major climbs off the calendar. The battle over the title of the French stage race with the GBF and Route de France organisers being at loggerheads with each other and ASO protecting their trademarks only served to make things worse, preventing a proper French focus to the calendar and limiting the importance of either race, while Spanish women's cycling was almost completely annihilated by the consecutive hits of Joane Somarriba's retirement and Maribel Moreno's doping case coming off the back of the overall hits to Spanish cycling from Operación Puerto and the financial crisis decimating the domestic scene. It took a decade for it to recover even to the state it is in now, with only the Basques putting on a couple of races and some underfunded but enthusiastic small teams like Bizkaia-Durango and Lointek keeping the fire alive for the duration of the 2010s until Movistar eventually stepped in.
There might be a few saw-toothed stages in races like Emakumeen Bira, and a few longer climbs in the Tour de l'Ardêche, but stages that would be classed as true mountain stages in women's cycling dwindled from being in a handful of decent stage races in the 90s and early 00s to almost zero in the late 00s and early 2010s. For example, for the year 2010, CQ records only 27 race days on the womens' calendar as being mountain stages. Of these, 12 are at the .NE status (most of which in the US), and some are in countries like the Czech Republic and Sweden which are hardly known for their high passes. Three are stages of the Tour de l'Aude (last edition), three are stages of the Emakumeen Bira, two are stages of the Tour de l'Ardêche, and four are stages of the Giro. That means that outside of those four races, there were literally THREE mountain stages across the pro calendar. That is something of a misnomer, as there will be some stages that included mountains but were not recorded by CQ as mountain stages, as there always are, but I think it's pretty illustrative of what we're working with here. There was no Tour de Suisse, no Vuelta, no Tour (only a couple of small races trying their damnedest to be a French stage race but often without too many serious challenges, and the Route de France would often have a Vosges stage as its only mountain stage), no Burgos, no Avenir, no Catalunya, no Romandie, all of which tend to have a mountain stage or two. Instead there was Bira (now Itzulia), the Giro, one stage of Toscana (which many riders boycotted after problems with traffic getting onto the course anyway), and after 2010 there wasn't even Aude. There was Trentino, but that ran inconsistently, being sometimes a one day race, sometimes a two or three-day stage race, and sometimes not running at all - but that's the only genuinely mountainous race that's disappeared while the calendar has been building up those other stage races.
As a result though, riders who were adept at climbing mountains AND had skillsets for the undulating/hilly terrain that proliferated around the calendar, such as van der Breggen or van Vleuten, had ample opportunity to develop and train at a higher level than riders who specialised solely in the mountains, as riders of that nature simply weren't valuable enough, year-round, to be invested in by the biggest teams, and that's often why you saw them flame out or be one-off successes, like Francesca Cauz or Kseniya Tuhai. I occasionally wonder what could have been with some of these riders. I've referenced before Eleonora Patuzzo, a young Italian rider who beat the likes of van der Breggen in the Junior Worlds and finished 22nd in Emakumeen Bira at 17, finishing 6th in it at 20, winning a mountain stage of the Giro del Trentino ahead of people like Arndt, Häusler, Pooley and Pučinskaite, but retired at 22 to pursue education. A rider with the skillset of, say, Pauliena Rooijakkers, would have been under-valued for those years (and indeed Pauliena herself spent half a decade toiling away as an occasional figure on Parkhotel Valkenburg before the parcours trends enabled her to become more of a valuable figure to higher tier teams) because there weren't enough races for her to use her best skills to make it worth investing the time and effort into developing her purely for that skillset. Riders like Cédrine Kerbaol or Marion Bunel, they're way more valuable than they would have been 15 years ago, and not just for being French or any comment on France's actions around a home Olympics, just based on their skillsets.
That process is changing. It is still in the process of changing, as we can see from the fact that the field still splinters to a very select number very quickly on these long ascents. There was a gap of 4'31 from PFP down to 10th place yesterday, and 9'23 down to 20th. But then, on Hautacam in the men's Tour de France, it was 7'06 from Pogačar to 10th place, and 11'47 down to 20th, and they have had plenty of chances to ride proper mountain stages for decades.