I guess miners who signal their support for BU do so, because larger blocks would be the better of two evils for them. SegWit would be good for Bitcoin and its users, but immediately lower the fees and thus be bad for the miners.
The problem for them is that segwit is a soft fork. That means that if ever it gets activated and 50% of miners applies it, it is FORCED upon all miners. However, BU is a hard fork. That is forced on nobody, as even a minority of miners can decide to go on the old chain. They need to express sufficient support for BU, so that segwit doesn't happen. And not sufficient support for BU, so that the hard fork seems without risk. So that they can keep the small blocks of today.
In other words, support for BU from a miner is most probably just a vote against segwit, not a vote for BU. But sufficient support for BU can make segwit impossible, which is the idea. And nobody will DARE to make a HF with less than 90% actual support, which will not happen either --> status quo, what the miners actually want.