Simple: Because, just as a lot of Trumplicans used to be conservatives and/or Tea Partiers who got fed up with the GOP "establishment's: inability to "FIGHT! FIGHT! FIGHT!" regardless of constitutional constraints and political reality and got fed up and defected to the first New York liberal conman who came along and told them everything they wanted to hear, so the GOP "establishment" and its conventionalist temperament will, ultimately, never fight off even a Democrat/La Clinton Nostra takeover that will extinguish it forever:
"The party will rally around Trump if he’s the nominee 100%”. Yay, team?
The same dynamic (an ironic term, that) is at work here as it was in all the Obama-congressional GOP confrontations over the past few years where our side didn't "swing for the fences" like TPers demanded, even though Boehner and McConnell were batting with toothpicks: business as usual. Just as Republican senators and representatives can't disabuse themselves of that clubby, "our friends on the other side of the aisle, "He's my opponent, not my enemy," Washington Generals-esque "peacetime" conventionalist mentality and realize that the past decade has been a Mortal Kombat war to the death for the soul of the Old American Republic, so they evidently are not capable of grasping that allowing Donald Trump to take over the GOP and turn it into a wholly-owned subsidiary of the Democrat Party - or, almost as bad, a twenty-first century American version of Partito Nazionale Fascista, complete with pointy white hats, available at the T.R.U.M.P. online store for an exorbitant price - would not only confirm the country's death, but their own professional and philosophical deaths as well.
We needed counter-revolutionaries; that's not the Republicans. Some of us knew this, didn't muster ridiculously high expectations and understood that they were doing the best they could, and there was nothing to be gained from vilifying them because politics is a zero-sum game and we could do a whole lot worse - because there is no one else; whereas some of us didn't know this, or wouldn't admit it, purposely misread the situation, deliberately drew the abysmally wrong conclusions, and became suckers and marks for the enemy infiltrator that will deliver the final death blow to us all by ushering in the 45th POTUS.
And that's that around which "the party [apparatus] will rally 100%"? See what Tea Partiers forced the "establishment" into? It's a self-fulfilling prophecy, not to mention a confounding source of "vindication".
Which is why we #NeverTrumpers will be sitting on the sidelines, doing our cardio to be able to screech "SEE, WE TOLD YOU SO!!!!!" in Trumplican ears for at least the ensuing four years. A third-party conservative run will not be necessary, because Trump will take care of taking his dive to the Empress all by himself.
And by the way - and I've been waiting almost a decade to say this - HUCK YOU, GOMER!
Exit quote: "Given an opponent who is disliked, considered dishonest, and facing trouble with the law, the largest plurality of Republican primary voters are eager to put up the one man in the country who is even more disliked, even more thought of as dishonest, and also facing trouble with the law. Way to go, Trumplicans! Give yourselves a hand!"
One more thought: Given Mrs. Clinton's commonly-known but NEVER-talked about health problems, and her likely choice of running mate, the world may wind up with two President Castros in the not-too-distant future. Seriously, Trumplicans, is Ted Cruz really a worse option that that?