Because Keynes supports state intervention in matters of money, and states like to intervene in matters of money?
That could be one of the explanations why states like Keynesian theory, but it doesn't explain why most economists do.
Who paying the most economist's wages? Same people who like to intervene in the matters of money?
I'd say that, outright corruption, if anything, is extremely rare. However, at least one prof, confessed that he didn't dare to publish an article opposing the mainstream view for several years, fearing that he may lose his job. But a bias in selection and self-selection of economists is what happens. It starts with public schools and ends with Federal Reserve research grants. Add intellectual fads into mixture.
Also is very easy to be wrong for long time in economics. Unique events, mixed evidence, political pressures. Not exactly a lab.
Which is why economics isn't fundamentally an empirical science, but rather a logical axiomatic one.