Because very very few people vote for them, we have a defacto two party system in that voting for any other party feels worthless.

~2010 due to a number of factors a third party (the lib dems) gained a significant increase in votes on campaign promises centred around free university courses for UK citizens.

As no party had a majority, a coalition was formed between the conservative party & the liberal democrats.

The Lib Dems went back on every single thing they had promised and essentially forced a new generation of voters to either pick conservative of labour.