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?
Lauda, when you firm up your opinion on Bitcoin Unlimited please post. theZerg (Andrew Stone) seems to be running the project and forum with help from several others. In particular Peter Rizun from
Ledger. A list of other members is
here. It seems to be an interesting experiment but still early days.