Yeah, this is a big mystery to me too. The GOP hasn't been able to nominate a presidential candidate who can formulate a cogent, complete sentence since 1984.
I also think it's no big deal to read well off a teleprompter. Any empty headed local news anchor can do better than the typical pol.
I think it is because they deal with businessmen and lobbyists, few of whom can deliver a speech either, and who kind of look down on those people who can, as “unserious.” But mainly I think it is because they can’t look at recordings of their own speeches and hear and see what is wrong. It is “ I am who I am, take me or leave me “ kind of thing. And they surround themselves with flunkies who won’t tell the emperor he has no clothes.