Why did the Republicans nominate a man whose promises they had no intention of putting into practice?
Two reasons:
1) They've been taught to obey the party no matter what (see RINO). They can argue *within* the party, that's acceptable. But when the party nominates someone, the rank-and-file Republicans are expected to vote for that someone regardless.
2) They've been taught that any Democratic candidate is not just incorrect but morally repugnant and evil.
The modern GOP could nominate Satan for office, and too many (but not all) Republicans would bend over backwards defending him and voting for him.