Why was Trump's run in 2016 treated seriously by the media, even from when he announced?

[Sir Mohamed]

He wasn't seriously treated initially, with only taking down JEB! as a game-changer. He was entertainment.

Some might argue none of it was all that serious until he actually won.

Yup, pretty much this.

There was even plently of talk whether he's a Dem plant working to assure HRC's win. This seemed to be somewhat of a credible claim given he once was closer to the Clintons and invited them to his marriage in 2005.

Generally, Trump was largely seen as a joke in public prior to his election.