Because those who dont allow such huge blocks in their client simply contine on the chain with their choosen maximum blocksize. Although there may be many compelling chains with diferent maximum blocksizes, it doesnt make much sence to using the one what has little support.
At least this is how I understand it, and it is step from today central planning to more freedoom for everyone with the associated responsibility - if you choose wrong blocksize value, you might not be using the chain most Bitcoiners are on.
If this is truly the case and we are talking about chains splitting because of votes, then BU is worse than XT. Then I'm also starting to grasp the idea behind BU. The question is who hired the supporters?
To the Politbüro and its hired cheerleaders - the totalitarian traitors of a libertarian project - BU is indeed the worst of all possiblilities. XT and BU already forced Blockstream core to promise to raise the limit (to 4 MB to get 1,75MB). There is more to come beyond this joke.