That is likely realpolitik calculus - if they say nasty things about Trump they score points with the incoming Democrat administration, score points with the political insiders in Washington and the Republican establishment probably don't care - it being entirely possible that they don't actually like Trump (Republican establishment != right wing voters). It is a pretty safe thing to offer an opinion on from that perspective. They'll make no powerful enemies.

Trump is missing a shield here that most presidents enjoy. International leaders would still say polite things about Bush the Younger despite him doing objective global damage by leading the invasion of multiple countries. My read on why is because they see a risk that people committed to his policy legacy still hold power in the US government.