Bear in mind Churchill hadn’t been elected Prime Minister in the first place. The 1945 election was the first election in TEN YEARS.
Churchill had never been an especially popular politician. He was booted out of office in 1922, later got back in, then was out again from 1929 to 1939. He switched parties on a number of occasions, giving some people the impression that he was an opportunistic weasel, which to some extent he was. His career was a litany of misadventures, including some of what could charitably be described as catastrophic errors. His ascension as Prime Minister in 1940 was not an especially popular one and happened essentially just because he’d been the big dog in the attack on Chamberlain. While he was rightly regarded as a war hero by 1945, I don’t think the British public had any illusions whatsoever that they were obliged to make him a peacetime Prime Minister. (Though they did so six years later.)
During the interim between 1935 and 1945 there was a sea change in Britain in terms of support for a welfare state, most notably for a national health service. Churchill was a great war leader, but his views on other matters were, by 1945, way out of step with Britain’s working class. He was firmly anti-welfare state, imperialist, and his personal character wasn’t terribly suited to campaigning - he was gruff and prone to making personal attacks on Attlee and the Labour Party that tended to bite him in the ass. He supported continued imperialism abroad, which the general public did not.
When you get right down to it, Labour won the election because they were percieved as being the party with a forward-looking view on things. The Conservatives, at least by comparison, appeared to be living in the past. People were sick of war and wanted a new order, so they voted for the new leader. It’s not so much that they were voting against Churchill as it was that they were voting for Attlee and a national health service.