Because political parties are focused on winning. How is this such an earth-shaking surprise? The Democrat Party does the same thing, remember the phrase "vote blue no matter who"? That literally means as long as the person has a D next to their name, just blindly vote them in no matter their character or policies because it's still better than a Republican.

Well newsflash, the GOP thinks the same way. They calculated that standing behind Trump was a pathway to winning, no matter his personal character or scandals. And it turns out they were incorrect, so now they're changing strategy. There's nothing unexpected or particularly extreme about this arc.