There are three reasons why not.
First of all, they can't stop him. The #NeverTrump attempts to dump Trump all fizzled and any buzz about flipping massive numbers of delegates at the Republican convention is only a flight of faulty fantasy. They are stuck with Trump the Republican nominee. Another thing is that the Republican establishment is stuck with the Republican voters. Trump is merely the carnival barker for that zoo. Here's a related observation from Jeet Heer at the New Republic.
The #NeverTrump movement has failed again and again because it refuses to deal with the root of the party’s problem: It’s not Trump, it’s Republican voters. In the short-term, there is nothing the GOP elite can do to stop Trump’s nomination. Fantasy scenarios aside, they are stuck with him till the election. In the long-term, if they want to stop future Trump-like candidates from rising, they have to fundamentally shift the party’s approach to race. Trump didn’t emerge from an immaculate conception, after all. His rise is the culmination of a half-century of racist politicking that started with the Goldwater campaign of 1964 and became codified under the Southern Strategy. Only by confronting and exorcising that racism can the GOP become a multiracial party, as the Democrats already are, and avoid the next Trump.
Sorry, but I fear that demon is already beyond exorcism. The nation is stuck with Republican racism for a long time to come.
Second, they may not really want to. Andrew Rosenthal in a New York Times op-ed asserts that Republicans share Trump's racism -- but just don't like to talk about it.
Far too many Republicans share this kind of racism and have for a long time. Trump has just dispensed with dog whistles and revels in his bigotry instead. But this is the party the Republicans have been deliberately and assiduously building for many decades, the party of division and intolerance. George H.W. Bush’s racist tactics in 1988 against Michael Dukakis — the Willie Horton ad in particular — seem almost genteel by comparison.
Today’s Republicans have stymied every effort at reforming immigration, at achieving true equality for women, at ending the scourge of racist drug laws and criminal sentencing rules. The Republican Party has generated a wave of laws designed to make it harder for black Americans and other minorities to vote. It’s not that Republicans don’t want to deport millions of Mexicans and ban Muslims from our shores. They just don’t like to talk about it in the open.
Third, courtesy of modern communications technology, Trump's depraved, racist rants are recorded for all to see. And watch and watch and watch some more. So when Sen. Mark Kirk first endorsed Trump, Trump's racist, bigoted comments already were recorded in video clips and available on the internet. Kirk cannot get away from that, even with an un-endorsement. And the Democrats are not going to be shy in hanging the politically toxic Donald "Old Beach Apple" Trump around the necks of Republican candidates for down-ballot offices. For an example of what's to come, check out this ad by American Bridge (via Greg Sargent at the Washington Post Plum Line).
So, folks, the old saying applies to Republicans: you can run but you cannot hide.