Well, let's examine that question, shall we? It's not a matter of cowardice as it is a matter of pragmatism. Anyone that has any desire to run for the GOP nom against Trump is aware of three pretty basic facts: One: Anyone interested in running against Trump for the GOP nom surely realizes by now that he beat a bunch of OTHER people who wanted to do the same back in 2016. Two: Should anyone even WANT to run, they would not only be running against Trump, they would also be (likely) be running against the GOP party itself which - whether they like it or not (and they don't) is supporting the guy who won against The Great Unbeatable Cellulite Goddess herself. That's a fairly significant hurdle. Three: Whenever you have someone that WANTS to get the nom from a sitting president - and I'm thinking Kennedy vs. Carter in 1980, they tend to irreparably split the party during the general election, giving the White House to the other party. To a lesser degree, you saw that happen in 2016 with Sanders vs. Hilla. Hilla, while NOT the incumbent, was The Anointed One favored by the Dem Establishment - and when Sanders dared challenge Her Majesty, it kinda split the party during the general election. I've said this before - heck, even today: We study history in order to avoid the mistakes made by others - and in this case, to predict likely outcomes in the future based on what HAS happened in the past. Now, you're free to call having enough intelligence to know what happens in certain situations cowardice - but you might as well ask why someone not jumping off the roof of a skyscraper isn't an act of cowardice as well.
Running for president is not the same as jumping off of a skyscraper...The lot of Repubs are cowards to question the status quo because their party elite keep them from running.Yep, cowards.
As much as you WANT someone to challenge Trump for the nom, it ain't really happening. As much as you WANT to think it's because of cowardice, it's not. I know - since you are older than I am - heh heh heh - that you remember the scorn Ted Kennedy got for weakening the Dems prior to Carter getting his butt kicked by Reagan. Were he NOT a Kennedy, what he did WOULD have been a form of suicide - political suicide, that is.