The Celestial Exclusion

Over the course of several thousand years, probably starting with the creation of the first mortal race, the boundary between the Celestial Planes and the Material Realm became more and more impermeable. This culminated in The Celestial Exclusion, which was the first summer solstice in over 2000 years when the gods could not enter the Material Realm. From that day on, all celestial creatures require some great effort on the part of a material creature in order to cross the planar boundary and gods in particular are nearly impossible to summon here.
However, the flip side of the impermeable planar boundary is that any celestial who is brought to the Material Realm would not be able to easily cross back to the Celestial Planes. But The Principle of Celestial Cohesion would still be working on them and would likely put a great strain on the planar boundary as it currently exists.