Well, if they crashed on a certain block then the operators of the client would notice and presumably update to a newer version of the client. The problem in this case was more subtle: the old clients did not crash on the block, they simply rejected it as invalid and continued working on an alternate subtree.

If you did somehow create a block that all clients except yours crashed on, you still wouldn't gain control of the network because the difficulty of creating a new block is calibrated according to the total power of the network, and only re-calibrates every few blocks. So for a while, you would still be effectively competing against the former power of the network, giving the client developers time to fix the crash.