UCI tends to make the extreme on the market, the rule.
Local unions, tend to make the rule, the exception.
When the UCI finally adopted the 29" wheel size for MTB racing, at any level, they had first consulted Gary Fisher (he has one of their gold master emdals at home, and named the MTB, so logical choice). Gary had little time, so I was allowed to pre-write some stuff he brought to Switzerland to discuss with the council. I made additional recommendation to further define the wheel size, to make is width-specific rather than the eternal max height. tire width is the whole deal anyway. MTB'er were not a hit BECAUSE of the smaller wheels. Those just happened to exist with fat tires for kids' bikes.
Of course, all the UCI did was change the 26 in teh rules in 29, keeping the "inch" as rule. Neglecting the fact that a 26.0" tire is smaller than what 99% of all XC worldcup riders used allt thorugh the 90's and beyond. rims are 22.5" across, you know, so do the match.
Less of an issue with 29" (rims exactly 25"), but I do race on tires taller than 2.00", naughty me.
After the acception of 29" in 2004, the Dutch Union decided to sneak in a rule change. One official had a grief with big wheels (old roadie style attitude), and was never too busy to express his discontent with the wheels' very existence. Officially, the union said, when asked, that "it had received concerns about the larger wheels being faster in a straight line". Well, DUH!
root cause of negative thoughts: racers had brought their all at once legal XC bikes to races. The fact that most had flat tires, and finished lower than might be expected of them, was conveniently overlooked.
Puritans just hate that riders get to choose the style of bike they want to run.
Rather than ruling out narrow tires (a more specific difference between road, cross and MTB bikes), they, especially that one active official, chose to handily ditch the 29" rule. The much-hated Nishiki 29" bikes brought in a -23 men Nationals title in 2004. A plain alu Nishiki frame, short rigid carbon Pace fork, and low-grip WTB naoraptor tires (mid-90's design), did the trick on one of the most challenging in the country (former Eurocup) course of Bergschenhoek, in the hands of Jelmer Pietersma. That much have hurt the conservatives.
The rule being in violation with the UCI rules though, was only valid for national races. Not even for UCI events within the Netherlands.
It took an outcry by the riders and team owners to correct this, per my original suggested, to a tire with of 40 or 45mm (I forget). The common feeling among riders were that while 29" bikes are totally g*y, pointless, and ugly, they at least are MTB's, and they hope allt he competiotion would run them, unless they'd get to a tight singletrack first. As everyone knows, there will inevitably be a slow march or pile-up.
How hard was that? Cross bikes get to run up to 35mm, and will rarely fit 45mm. MTB's must run much wider. There cannot be a misunderstanding.
My opinion : there are better things to keep out of racing. Like carbon wheels. too expensive, but a definitive edge for those who can afford them. As wheels can be swapped out, it gets even more expensive. Carbon wheels and finicky technology is one big reason 'cross is such a small sport in terms of numbers of participants.
No-one read all of this, but thanks anyway.