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?
Best Answer
This answer reads in four sections:
Background
The answer to this question appears to be tricky, as there is a need for the players (and hence mortals) to be at the center of the gaming world, but at the same time there is a desire to design ever more challenging adventures for them (stopping evil powers from invading your home world is challenging). Moreover, Forgotten Realms being a very large setting that has had a special standing since the days of TSR, with many authors and game designers contributing to it, there are occasional lapses in continuity, which are retconned later.
For example, over the many editions of D&D, Tiamat had realms/residences in various Outer Planes, became a servant to Bane and Asmodeus, had three different aspects, and finally became to be the subject of the module Rise of Tiamat. Since this created confusion, folks at candlekeep.com asked the opinions of Ed Greenwood, the originator of the Forgotten Realms setting, and also the first person to have described the Nine Hells within D&D context (in two articles that appeared in the Dragon Magazine in 1983, articles in which he had mentioned Tiamat as the ruler of the uppermost layer, Avernus). You can read his full answer in a post from Feb 2015 on Candlekeep.com; it is a long answer mentioning the aspect of Tiamat from Rise of Tiamat explicitly at some point, so I suggest you have a look at it to put that module into context. The short part I want to quote is about how even the most knowledgeable designers, even Wolfgang Baur of Kobold Press, who published the module itself, could not see the full picture:
In short, it is possible that the designers or the product commissioners might have missed thinking about the serious issue raised in this rpg.se question.
Answer from Lore
The 2e Planescape product On Hallowed Ground, is a 192-page book dedicated to deities and their servants in all campaign worlds of the AD&D game in 1996 (which included Forgotten Realms). In it we read (page 40):
Hence, it seems like even when a crazed deity like Tiamat wants to enter the prime material plane, she does not enter on her own and need to be "invited". While other deities would not like that to happen, they need to trust their own servants; mortal heroes can be as effective as their avatars. This is the way one needs to abide by if one does not want an arms race that will ruin the prime material plane.
Regarding Bahamut
Since the question mentions Bahamut in particular, it is worth mentioning that there is a clue in the 5e Monster Manual (page 103):
Perhaps Bahamut is already playing a role behind the scenes, but being a deity of law, he might be holding onto the rule/agreement of not interfering directly, realising that in the very long run, this would harm the prime material. Besides in the grand scheme of things, who can say that a new "age of dragons" would be a bad thing for Bahamut in the long run? As the evil dragons start ruling the lands, there is a chance that there will be more worshippers who would appreciate the Platinum Dragon.
5e Forgotten Realms Specific
Finally, with the rewriting of the Tablets of Fate by overgod Ao, the tone of the Forgotten Realms past the Second Sundering is claimed to have somewhat changed. According to the Sword Coast Adventurer's Guide (page 18):
How subtle the signs are and what quiet really means is difficult to assess, given the scale of the disaster that needs to be avoided at the end of Rise of Tiamat. Different contributors to the setting seem to have somewhat different takes on this. For example, the article about Eilistraee on the FR wiki refers to the novel Death Masks by Ed Greenwood and states:
So seemingly casual dancing and talking to mortals is possible for at least some deities. Perhaps they are "subtle" in the sense that the avatars, like the venerable human avatar of Bahamut, are just talking now, and expecting that the mortals take the action. This interpretation appears mostly in line with the description provided by Matt Sernett and Chris Perkins in the Dragon Talk podcast from December 2016 (episode 'David Eddings on Roleplaying Claptrap', after the 8th minute):