Why would they want to fork off if they weren't going to succeed?
Because they'd like to see a chain with larger blocks
This is not an explanation
What is the purpose of seeing Bitcoin with larger blocks if no one is going to use it anyway? As to me, it makes sense to start this coin only if they really intend to come out as winners. Regarding the Bitcoins you mentioned, I guess that they hadn't enough support by the mining cartel. Anyway, the failure or success would greatly depend on the hash power but indirectly. Hash rate itself is meaningless if you don't have users, but it may make all the difference if you are going to take down the competing chain in a really ingenious and sophisticated way