I am not a historian, but I am a "layman" that is very interested in elections and the electoral process. The main reasons for Labour's win in 1945 basically boil down to:

1. Churchill was personally popular, but the Conservatives as a party were not. During WW2, both major parties governed in a combined Joint Wartime Coalition, agreeing to put aside their partisan differences for a united government. Churchill did not lead a Conservative Government, he led a coalition of both Tory and Labour members. So Churchill was not able to translate his personal popularity to the Conservatives more generally.

2. With the war coming to an end, a huge amount of post-war reconstruction was needed. Millions of people were displaced, houses were destroyed, discharged servicemen would be coming home and needing jobs, homes, and support. Labour proposed quite a radical plan of social reform: "full employment", creation of the welfare state, big government spending on infrastructure and rebuilding, and so on. This was exactly what the public wanted.

3. In contrast, the Conservatives' campaigning was focused more on Churchill personally, without making much of a plan for the future. But as I said in points (1) and (2), this was completely the wrong strategy.

4. The Conservatives had been in power for most of the preceding decades. People still hadn't forgotten the policies of Appeasement, or the difficulties of the Depression years, most of which occurred during the Conservatives' watch.

TL,DR: with the war over, people wanted to look forward and not look back. Labour simply had a bolder and 'better' plan for post-war reconstruction and social reform.