Let me explain in market language not technical, only a big mining farm which also are manufacturer of mining machines are supporting BU.
BU will allow miners (or rather allow themselves
![Cheesy](https://bitcointalk.org/Smileys/default/cheesy.gif)
) to generate blocks with any size depending on the numbers of unconfirmed transactions in the mempool but they didn't just change that NO they have changed other things as well and that's why nodes were operating on BU software were attacked/crashed/exploited. (ever since ASIC miners came to be no longer 1CPU or 1 computer was a full node and now you could operate 10K ASICs on only one full node but that is about centralization topic and doesn't belong here).
SW(segwit) will allow the developers to implement major changes in the future without needing to perform a hard fork the same hard fork BU is trying to perform which will damage the whole network greatly if not 95% of all nodes/miners vote on it and switch to run BU software.
If you think SW will give advantage to anyone running SW activated software then you could as a miner/full node operator run SW activated software to have the same advantage, why do you refuse to see the logic in that?
But one can argue why not switch to BU and have whatever BU supporters are going to have? answer is simple: only ONE miner which has a great hash power is mostly in support of BU and as well BU software is not the same as the software which right now majority of the network is operated on.
If BU had only changed the block size limitation we'd be gladly accept their version but why there was some attack on their nodes and why didn't the same attack work on Core nodes?
Having freedom over the size of a block means they want to collect as much fees as they can in a single block without contributing computing power required more than of mining a 1MB block, so only miners with most of the hash power will gain more profit and small miners will remain the bottom feeders with no saying in the future of and if any changes.
What happened with previous hard fork(s) why don't we have several chains? because everyone switched to Core and followed Core rules.