I'm running the Rise of Tiamat campaign, and a fundamental question about when gods do/can and don't/can't interfere directly in mortal affairs (particularly via manifesting avatars on the Material Plane) has come up:
Given that the Cult of the Dragon is on the verge of summoning Tiamat, why don't other (good) gods interfere more directly to prevent/counter this?
For example, in a campaign about the summoning of Tiamat the Goddess of Evil Dragons, nothing in the book mentions or supports trying to summon Bahamut the God of Good Dragons (who unlike Tiamat isn't trapped in a particular plane) to stop her.
The book definitely expects that Tiamat is the only god that will be summoned/will manifest in or after this campaign. The possible aftermath of failure is described under the "The Horror of Defeat" heading on p. 88 of the adventure:
A victory for the Cult of the Dragon is a real possibility in this adventure, and would be catastrophic for Faerun. With Tiamat ascendant, the age of mortals comes to an end and the age of dragons begins. Nations and kingdoms shatter, civilization collapses into bloody war, and chaos reigns supreme.
It's clear from this epilogue that what doesn't happen in the event of Tiamat's Rise is a bunch of godly avatars showing up, telling Tiamat she's broken an important rule about staying out of the Material Plane, and de-summoning her.
Is there a canonical reason why gods (good or otherwise) don't tend to manifest avatars in the Material Plane in times of crisis/when they really want something to happen? And if there is, is that reason consistent with Tiamat being summonable to the Material Plane without other gods stopping her?