There is a number of reasons for this, but one is that if you hire someone external they are more under your control than an internal hire who already has ideas of how things should work and relationships and therefore has their own amount of political clout. A new hire has no reputation to lean on so must be a bit more cautious at first.
Another reason is that there is a “grass is greener” mentality. You know what you like and don’t like about an internal hire. You don’t know what you like or dislike about the external hire yet, not really. Additionally, someone from outside may have the silver bullet to solve X,Y, or, Z problem that no one else can seem to solve. You know your internal candidates don’t have the quick easy solution you want, but maybe this new person will!*
* hint: They don’t.