Trump, perhaps more than any other human being in history, single handedly made them feel good about being human garbage. We were making social progress, effectively shaming people who were bigoted and hateful, and they resented it, but were starting to hold back in public and even maybe become a little less shitty.
And then the shittiest, most ignorant, most loud mouthed unjustifiably confident hateful pieces of human garbage rose to the top. He was able to get away with saying all the things they’ve been shamed out of saying. He vindicated their feelings, pushed back th tide of progress.
You’re right that the republican leadership could’ve got what they wanted from another republican. But they stirred up all of this hatred and ignorance but at least gave a fig leaf of cover and used dog whistles and such. When Trump came along and said the quiet part out loud, he hijacked the machine they used fox news/facebook/etc to create. They couldn’t out-extreme Trump, and the inmates took over the asylum.
They thought it was going to sink them - Trump brought everything they were trying to do out into the open and more extreme, and they thought the American people would reject this. They underestimated the evil and ignorance of their own base. And then when they realized that Trump wasn’t going to alienate their core base, that they were such garbage that they actually felt extreme personal loyalty to him, they decided that’s the future of their party and tried to out-crazy each other to keep up.
Anyway, that’s why Trump, and not some generic republican candidate. In 2016, they wish they had a generic republican candidate, but after that - after Trump opened that door and found out just how low the republican base can go - they’re all in on this road they’re currently on.