Because they don't want to be an Altcoin, they want to be Bitcoin and think that BU is Bitcoin. Bitcoin allows changes to its protocol and they see their change as such. Different developers and clients are ok. I learned that the longest valid chain and the biggest hashrate usually correspond. This is why they have a point to be the Bitcoin in case of a fork with over 50%.
Others view it as a attack where you gain +50% by playing by the rules and immediately after it changing the rules.
And frankly right now this is a political thing and both side sees the other one as threat to Bitcoin so they fight to "kill" the other idea/course of action.