Labour are doing badly as the moment with polls indicating that Conservatives have up to a 21% lead. Calling an early election at a time when they are in a position of strength favours the Conservative party. It seems most likely that Labour will do badly and Conservatives will gain more seats in the general election.
The early election needs support of two thirds of MPs to go ahead as per the Fixed-term Parliaments Act 2011. Labour largely voted in favour and the vote has passed. They could have blocked it by themselves as they have 35% of seats. Why did they vote in favour of an early election which is probably going to go badly for them?