Why did the mainstream media push the Trump-Russia collusion story so hard?

It's not a mystery.

Writing in the Wall Street Journal, Holman Jenkins says:

One day it will behoove science to dissect the brains of journalists (ideally post mortem) to find out why they thought it was a good idea to promote lies about Mr. Trump [i.e, the claim that he colluded with Russia] when his life and business afforded a bottomless well of defamatory material.

That’s good writing by Jenkins, but no dissection is required to discover why journalists ran with the Russia collusion story. In fact the cause is over-determined.