It is now mid-March, at this writing, and the G.O.P. feels that there is enough time, until the Republican Convention in July, to undo the primaries and nominate an alternative, to Donald Trump, to be the Presidential candidate of the Party. At the same time, there is not enough time, before the elections, in November, to even consider a nominee for the Supreme Court.
Republicans explain that an appointment to the Supreme Court is an act of governance, whereas the change of candidates is part of electioneering. And as everyone knows, Republicans suck at the former but excel at the latter.