Because if they do, they won't be able to spend their coins anywhere, or sell them, since nobody else is running their code. Their coins will be worthless and they'll have wasted their money mining them. It makes no difference whether they have 51% or not. Fortunately, miners are smarter than OP and won't do this.
None of this is necessarily true. Part of what gives Bitcoin it's value is it's security, and if a large amount of the hashpower were to suddenly stop securing the Bitcoin network, then some people may wish to join a network that has a higher amount of security (e.g. the network that has 2MB blocks).
Furthermore, the economic majority is in favor of raising the maximum block size, so it would be likely that most Bitcoin related companies would use the 2MB branch of the fork