Hashing power is only a piece of the large puzzle that makes up a successful fork.  Ultimately, it is about the exchange rate.  Every fork will have its own exchange rate.  Whichever has the higher value, people will migrate to.

Here's an interesting scenario.  Create a fork with no more block reward.  Given we are at about the half way mark to 21M, it should be worth double the value of the current bitcoin version.  Key word should be.  It would definitely get people's attention.