why don't miners make a mind to either support btc or bch?
they have made up their minds. they support more money!
all miners are like this, it is not just bitcoin miners and it is not just bitmain,... that people like to repeat. every miner (even solo miners at home mining altcoins with their GPU or CPU) are always looking for a way to make more profit. when something is giving them more money they will switch there.
BCH was temporarily like this, they could potentially make a lot more so they switched a part of their power there and then back again to bitcoin that matters.