That's because most of them were in the Shadow Cabinet and Harman whipped an abstention in one of the most idiotic political gestures of all time. Corbyn could vote against because he wasn't in the Shadow Cabinet at the time. The resignations actually encompass a wide spread of the party - Malhotra is very much on the left of Labour. This is not a Blairite coup.
Yeah, all true, though I believe you can vote against the Whip. And also true it's not just the Blairites who are against Corbyn. But what are the bets it's a Blairite who is the replacement, and that all the talk of Labour being "unelectable" then vanishes? It's hard to not draw the conclusion (from this, and the initial leadership contest) that what the PLP believes will get them elected is actually what lost them two elections, and that they're now just doubling down on being a centre-right party.
Edit to add: Which is fine, if there were a centre-left party that was actually decent. The Greens are... meh, and the Lib Dems are a wash after the spinelessness of Clegg - it'll take a lot to wipe the memory of tuition fee promises from people's minds.