One problem is space for warming the dishes. Ovens can hold only so much, and if several people add a dish to it for warming there is likely to be a problem fitting things in. Yet all dishes served at the same time must be heated over the same time. Pies are eaten as dessert, so even if a pie is preferred to be served warm there is time after the other dishes are removed to place them in the oven.
Another problem is food can spoil if left at the wrong temperature too long. This is a concern in transporting many dishes. But, pies can be eaten warm, cooled in a refrigerator until needed, or even eaten, and for many pies left at room temperature for the day. So safely transporting most types of pies is easier in the safety sense.
Pies are easy to transport, with less danger of spillage. There are not small bits of food to fall from a dish as it bounces along a highway.
And, pies can be placed in throw away aluminum pie pans for the meal. With other dishes there are often leftovers, and that either means leaving a bowl or tray behind, or wasting food that exceeds the storage containers available or the capacity of the refrigerator. If the pie pan is a throw away it will store leftover pie just fine, and will not cause a problem of later retrieval of it.
So, if you do not have a special dish that everyone waits for, and no one else has claimed pies as what they are bringing, pie is an excellent option. And, if there are leftovers, it is something the host and hostess are more likely to be able to finish before it goes bad.