Because they care about the health of the network, they don't want to fork because they know that's bad for bitcoin... Once BU has a large majority of the hashrate (75% IIRC) they will start producing large blocks (and thus fork) in the hopes that the other 25% will then have no other choice but to also start mining (or atleast accepting) big blocks... They know that if they fork now they'll lose all the hashrate they currently have and that they'll create 2 BTCs, which they know would be horrible for adoption. Just imagine new people learning that there are 2 bitcoins all of a sudden