Mass is something required to be held each Sunday if at all possible. It’s not always convenient to have them inside for different reasons, such as remoteness, renovations to a church or what have you. But the Sunday Mass is still a necessity, so occasionally but rarely, when there’s no other alternative, they are held outside.
Weddings, on the other hand, are never a matter of this sort of urgency, and are always planned well in advance, months if not a year or more. So there’s never any compelling need to have them outside. It’s not like you can’t find another church if yours is unavailable, for example.
