Mainstream media bias, plain and simple.
I know it’s a cliched term at this point, but when I say “mainstream media”, I’m referring to the big 3 cable news networks (CNN, MSNBC, and Fox), the big 3 broadcast networks (CBS, ABC, and NBC), and the two largest newspapers in the country (The New York Times and the Washington Post). These outlets dominate most news coverage in America, and all of them (except Fox) are left-leaning in their coverage (sources: AllSides, Investors Business Daily, Harvard, etc.
Although Republican presidents receive more negative coverage than Democrat presidents do (Bush was 57% negative vs just 41% for Obama), Trump has been the overwhelming target of relentless negative coverage since he was elected (80-95% depending on the time frame).
Put simply, Democrats and most of the major news outlets agree with each other ideologically. That’s why you don’t see a lot of criticism of Democrats’ hypocrisy. It’s an error to rely on their coverage to formulate your perspective on the matter.
If you scope your search to more right-wing outlets (Fox, National Review, The Federalist, the Daily Caller, the New York Post, Washington Examiner, etc), you will undoubtedly see the criticism cut in the other direction. These news sources, however, make up a much smaller share of American news coverage.
On a side note, all politicians have an agenda (it is, after all, their job). Ironically, they are all quite principled on whatever that agenda is. Unfortunately, scant few are actually honest about it, preferring to make an appeal to whatever principled reason is more politically convenient at the time to justify their position.
In 2016, Republicans didn’t want Obama — a President they made it their mission to obstruct — to nominate what they perceived to be another SJW judge to SCOTUS. Full stop.
Mitch McConnell didn’t have to waterboard everyone with some BS excuse about nominees in election years or whatever. All he had to do was say “No” and leave it at that.
In 2020, Democrats don’t want Trump — a president they despise — to get another bite at the SCOTUS apple. It’s sure fun to point out Republicans’ hypocrisy this time around, but we need to stop pretending it would be any different if they were in charge.