Because I can't vote for the government, Naomi, for all kinds of reasons. Their policies are causing very serious and unnecessary hardship to the disabled - including people close to me. I also think May has an extremely dangerous attitude towards policing the internet (which is already yielding terrible results) and that her government is propped up by interests which guarantee profound societal problems that the Conservative party is actively prevented from addressing in a way that Labour isn't.
And because of our broken system, if you can't vote for the government - you either don't vote, or you vote for the other lot.
So, yes, unfortunately Labour are the lesser of two evils from my point of view. Even if they were not, though, voting against them would still be utterly pointless for me because I live in a safe Labour seat - so any vote that isn't Labour would be actively wasted, and if I voted for an alternative that I liked better (say Lib Dem), I would actually decrease the chances of policies I like happening rather than increase them. Look up the "spoiler effect" for a more detailed explanation of what I mean.
So, much as I dislike the current leadership of Labour, my only other choice to voting for them is not to vote. :/