Because the UK, like most sensible nations, doesn't force voters to either choose which major party they hate the least or to throw away their vote by voting for a party they actually agree with but has no chance of winning like they do in the US.

Instead they have a ranked voting system where you number the parties in order of preference like we have here in Australia.

The party you ranked number 1 doesn't have enough votes to win? Doesn't matter, your vote goes to the party you ranked number 2. That one also can't win? Then your vote goes to the party you ranked number 3, and so on.

This gives voters a chance to vote for the parties they actually want to win, and ensure those parties get their share of government funding, while still having their vote count towards whichever major party they judge to be the lesser of two evils.

This helps smaller parties gain much more traction than in the US voting system.