Trump is getting pushback because he's a horrible candidate in comparison to most of everyone else who ran in the primaries...
But... he's the guy who one, and that's something a few GOP elites still have a hard time facing because it's their failure. The egos of 17 people collided, and not everyone was smart enough to get out of the way early. Perhaps the GOP as a party could have done more to address that some transparency and without stooping to the level the DNC did with Sanders, but it all came down to egos.
To me, Trump's nomination is as seismic of an event in GOP as it was for the flood of Tea Party candidates who were elected in 2010-2012. The establishment can't survive this, and the GOP will be better off for it once they realize that the days of old are gone.