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Because the manufacturers will insist, due to consumer demand. More and more people are moving to discs because the benefits far outweigh the disadvantages. Pro cycling is little more than an advertising vehicle for products so, ultimately, the manufacturers who sponsor teams will hold sway.
Wait until a rider wins a stage of a race due to being able to descend faster in the wet because he has disc brakes, then you'll see more pros making the choice.
The UCI have been luddites for too long. There is no other sector where they've moved to hydraulic disc brakes then gone back, as the performance is superior to existing braking systems. The manufacturers, who pay a lot of money to get teams on their bikes, want it, and consumers appear to want it too (take cx as an example, discs are used sparingly by the pros, but at a local league level the majority of riders are now on discs). Ultimately pros will have to adopt them, so the CPA (which is toothless) should either engage constructively or pipe down.
Ventoso's injury was nasty but there is only his word that it was caused by a disc and even then he doesn't know for certain. Given the description of the incident and the location of the injury, it could've been caused by something else. If rider safety is key, and it bloody well should be, then I'd expect the UCI to look at everything related to it, rather than making a knee jerk reaction to some negative publicity. That was how McQuaid and Verbruggen operated, some of us were hoping for a more considered approach from the Cookson regime.