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. Bitcoin XT, Bitcoin Classic, Bitcoin Unlimited and Bitcoin ABC have all basically failed, despite all the hype they each had for doing the same thing as last time, so the only solution is to fork off without a majority of hash rate.
Major groups backing BU/EC have decided that a compromise is necessary, but they've decided that they want their own chain to play with.
You could equally argue "why did they start Clamcoin if they weren't going to take over BTC?"
The big block advocates, or rather this strain of them as I don't really have a problem with larger blocks, seem to operate in something of an echo chamber of their own construction.
I'm not sure I subscribe to this rhetoric of just bashing the people.
This is absolutely a self-fulfilling prophecy, because Roger Ver is not a coder. He's just an opinionated public figure. So by bashing Roger Ver rather than big blocks, you're keeping yourself from joining the "big block" side. And by doing that, you're stopping any "good" people from moving over to that side, and keeping the majority of people annoying. It's like when people avoid going vegan because they don't like vegans, or avoid using BTC because it's used on the dark net.
So more than anything it's an echo chamber of everyone else's construction.