BP can be seen as the opener of the Ardennes classics, most rides from Itzulia ride these Ardennes classics. So they would like them to also ride BP. They are afraid that if they make BP harder, less riders will want to do it because AGR is 2 days later. Since AGR has more prestige, riders wouldn't go all out or skip BP if it's too hard.
Sure, it
can be seen as the opener of the Ardennes classics... but it by and large isn't.
Realistically there's something of a continuum of progressing difficulty of climbs in terms of length and so on from the northern Classics through to LBL, and with the move of AGR away from the Cauberg finale (and the increase in prominence of the miserable, worthless Bemelerberg at the expense of actual good climbs like Keutenberg) you could see the period as being two "pairs" of races, the 'intermediate' hilly classics of BP and AGR forming a bridge between the northern Classics and the 'true' Ardennes of Flèche and Liège.
That was what I
thought the BP organisers were heading towards, and it seemed to be going well, because especially with the most recent AGR routes, it could attract some of those classics men who don't specialise in the longer hills so would be unlikely to compete on the Mur de Huy, but can power over the 400-500m length ones. BP can very much serve as half of a double act with Amstel Gold, with the profile of the climbs matching up with many of those in the Limburg hills.
But what BP isn't really preparation for is a series of 2-3km steep hills as the Ardennes tend to provide. That isn't to diss the race, it's just not what it has at its disposal. The Volta a Catalunya isn't having several flat stages with cobbled roads to encourage riders to use it as Paris-Roubaix and RVV preparation either. Most of the climbs in BP are the kind that we joke about being "secret" climbs that go unnoted in the Itzulia roadbook because, I don't know when the last time you looked at a topographical map of the Basque Country was, but it's kind of geographically not very similar to the Brabant at all. Which then reflects in the kind of riders it attracts, so trying to attract the riders from Itzulia to BP is kind of a fool's errand, especially trying to attract them by making the race
easier, i.e. easier for sprinters and rouleurs to survive, and harder for the grimpeurs and puncheurs that typically make up the lion's share of the stage winners and GC field of the Itzulia to make a difference in. If you
do attract them through that, it's more likely to be as logged miles ahead of the Ardennes week proper and the races that suit those riders, not to add anything of substance to the race.