If you have a subscription to The Wall Street Journal or feel like signing up for one of their free subscriptions that permit very limited access, you might want to read their article called "How Trump Lost the Midterms." It contains a lot of reasons, all of them probably valid to some extent.
I could add on a dozen more, mostly about Trump increasingly coming off as unstable and so untruthful as to raise doubts among even some of his most devoted supporters. Most of the reasons the article gives fall into the general category of Refusing To Listen To People Who Knew Better, especially about Iran.