Because Trump has little idea of what conservative/GOP values/principles are and has repeatedly shown that through his statements and positions. There's a reason why Pence had to slightly contradict some of his stances during his debate. Not to mention the GOP being the party of "morality" and Trump literally being an amoral, awful human being. Then you have his foreign policy positions (what few there are, none of which he seems to be able to consistently nail. Case in point his arguments for nuclear proliferation) which are so bad that conservative foreign policy experts have abandoned him in droves to either endorse Hillary, who is ironically enough more in-line with their beliefs, or refuse to endorse anyone at all.
Trump is nothing of what the GOP has presented themselves to be the party of for decades. He is however, a fantastic representation of the Freudian id of the GOP as well as the result of what happens when you have nearly a decade of a party without a purpose beyond "Stop Obama!" and one that directly contributes to government dysfunction in that pursuit, pissing off literally everyone including their own supporters in the process.
They may have spent the past 8 years making Hillary into the "Great Satan", but Trump is their Greater Satan and at this point rational conservatives have resigned themselves to back the lesser of two evils. So have a whole lot of Democrats that aren't thrilled with Hillary.
Basically, it's the GOP's fault they're in this position right now. 100%
Edit: And while I've gained a modicum of respect for Beck for his positions against Trump and am thrilled to see him resign, he's still largely a piece of shit that contributed to the current state of affairs in a major way by mainstreaming conspiracy theories through his show on the radio, on Fox News (lol), and now through his site. His skin is in the game and he can't bow out now without being covered by the same filth that covers those he's condemning.