Because, until recently, Rupert Murdoch decided who would win which election. It rarely has anything to do with policy, and almost always is Hobson's choice of "best of the worst".
Labour in 1997 were Tory Lite - I didn't vote for them after 1997 until 2010 because I thought Brown had the best plan to get out of the global financial crisis. I still think he had the best plan.
I couldn't vote for Miliband, and I didn't think I could vote for Corbyn but then I read his policies and he got my vote, and will get it again in the next election (February/March 2018).