This is a really fascinating question that British political historians have considered quite extensively, and so any Reddit answer will fail to adequately cover all aspects of the question – for a wonderful narrative of twentieth-century politics in Britain, have a read of Peter Clarke’s 'Hope and Glory' (2004). Indeed, like most elections, the result of the one in 1945 was influenced by a number of different factors. Some of these included the relative unpopularity of Churchill compared to that of Attlee as a political figure and the vision Labour espoused, along with a notable cultural shift that took place during the war that made traditional Toryism appear inappropriate to the time.

In spite of common assumption, Churchill was hardly as popular as people tend to imagine. Prior to WWII, he had earned a reputation of extreme, hard-right liberalism, traditional jingoist and antiquated imperialism, and being an extremely difficult person to work with. Of course Churchill joined the Conservatives having previously been a leading figure in the Liberal Party until 1924. Working as Chancellor of the Exchequer under Baldwin, Churchill paved a way to political isolation by calling for military action against miners in the 1926 General Strike and using particularly aggressive rhetoric. His vehement opposition to tariff reform and Indian home rule further ostracised him, consolidating a position separate from mainstream politics in what is known as his ‘wilderness years’ – Churchill was shown again to be alone when he was not invited to join MacDonald’s 1931 National Government Cabinet.

It was only Churchill’s staunchly critical stance on appeasement that provided him with a position in the war government, which notably was a cross-party coalition and not a Tory government. Churchill’s following and perceived oratory ability guaranteed him a place in Chamberlain’s War Cabinet, and when the Prime Minister resigned, the recommendation of Churchill as a unifying figurehead led to his acquisition of power. But during the war it must be noted that Churchill experienced a growth in popularity, but significantly less than is commonly asserted. Churchill only made a handful of radio speeches, usually hours after far more well-received sermons to the Commons and several drinks – oral histories seem to show that people’s original thoughts were more ‘Oh, it’s him again’ rather than the patriotic euphoria that has emerged as national myth since. Indeed, speeches from other Cabinet ministers and figures – such as certain Anglican priests who spoke more on the topic of reconstruction and the war for a ‘New Jerusalem’.* The cultural shift that took place during the war thanks to such speeches had a profound impact on the debate that the election was about: modernisation and reconstruction vs. a continuation of the perceived poverty of the 1930s.

On a side note, people appear to believe they heard the ‘We shall fight on the beaches’ speech on the radio that evening, though unusually no recording took place until after WWII, showing that much of the cultural memory of Churchill has been influenced by events since.

Thus, Churchill’s wartime appeal has been grossly exaggerated. In fact, memories of his record at Gallipoli and 1926 had a more of an influence on his contemporary image than his wartime experience, as the 1945 Walthamstow Stadium Speech demonstrated where he was heckled by a crowd of 20,000 as some waved hammer and sickle flags. Similarly, Churchill’s chief electoral message was a boast about his wartime record, which again was marred by reports from government and military colleagues about his alleged drunken tirades and one-track mind-set. His outspoken criticism of Gandhi throughout the 1930s and even during WWII led many to view him as what could be described as a racist, imperialist bigot. Compare this to the message of hope espoused by a mild-mannered and optimistic Attlee; Churchill's complacent references to him in public as a ‘modest man with much to be modest about’ seemed to show how out-of-touch the PM was. Attlee had a strong record of philanthropy in London’s East End as well as a long history of attacking successive governments’ records on poverty, an issue that found itself the centre of electoral debate in 1945. Attlee had separated himself from the 'Guilty Men' responsible for pursuing appeasement with his calls for limited and, later, full rearmament by 1938. Again, Attlee's wartime record showed himself and Labour to be both collaborative and competent; with War Cabinet figures like Bevan and Bevin (who happened to ironically get along poorly) taking up important roles in the future Labour government. Unlike Attlee, Churchill’s support of violence against strikers and references to socialism (and anything that resembled it) as the biggest threat to Britain, prevented him from shaking off the shadow of his privileged background. His anti-Bolshevik stance undermined the Tory electoral promises of welfare, making them seem less believable than that of Labour, which again crucially counted against his campaign.

Further, the Conservative election posters were notoriously bad, with slogans like ‘Let him finish the job’ appearing out of place in a society scarred by memories of a harsh 1930s and PM rushed into power in a time of war. Labour’s images of reconstruction, such as the ‘Vote Labour for…’ series (featuring policies matched with archetypal members of society, like the elderly, the soldiery etc. to appeal to a broad voter base) were far better received, on the other hand. While Churchill's patriotic speeches may have been appropriate in a time of war, they felt inappropriate for peacetime rebuilding.

The 1945 election result was - even in the hopeful Labour camp - unexpected. The astonishing 11.7% swing resulted in a gain of 239 seats - a world away from the humiliation experienced in 1931 and slightly-improved result in 1935 (of 46 and 154 seats, respectively). The landslide came from a combination of Churchill and Tory shortcomings, cultural changes to favour the hopeful vision expressed by Labour, and the popularity of Attlee as an individual. The complexity of political change, along with the inaccurate cultural memory of Churchill’s wartime popularity, cannot be fully explored in a short answer, so I’d recommend 'Hope and Glory', E.H.H. Green’s works, and Rosenbaum’s 'From Soapbox to Soundbite' for a far more in-depth investigation of British electoral history.

I hope this has helped answer your question - the documentary that /u/muonsortsitout mentioned provides a fantastic overview as well!

*I currently cannot remember the name of the most famous such priest but will amend this ASAP.