so why are we talking about fork leading to two versions of bitcoin, if BU gets 75% concensus then logically BU should be the only bitcoin and the other fork should be killed, just the mining logic of adding to the longest chain.
No, it's not logic, Chinese miners may not be proceeding in good faith, bitcoin is widely used in China to evade capital controls, that is not something the Chinese government is very fond of, so Chinese miners may be being subsidized to take more control over the network in order to control transactions.
This is the kind of state sponsored attack we've been talking for years, if more states around the world begin to subsidize bitcoin mining we might be able to compete with Chinese and keep bitcoin decentralized.
BTW, only 40.6% of hashrate is signaling for BU and only 11.32% nodes are BU, can't see where you get the idea the majority supports BU...