Depending on the source, though, that's either for essentially "Lawful" reasons in that he was getting above his station and challenging the authority of the gods, or because of the "Doctrine of Balance", which is a very vague and wishy-washy Krynnian concept from the 1980s, one which is rarely mentioned and like all other TN ideas in D&D, never really explained, let alone fully explicated with reasons and conditions.
The closest I think we ever get are in two sources. First is the explanation of the Law of Gilean in
Dragonlance Adventures; while I don't have my copy to hand at the moment, it talks about a need for contrast and 'unity in diversity.'
Second is the material on souls' progession in
Holy Orders of the Stars, which I do not and have never owned, but which I believe states that the 'gods' are embodiments/examples of fundamental principles that are all necessary for souls to progress to a higher state of being through the full experience--good and evil--of mortal life. (At the risk of bending ENWorld rules, you can see parallels to Hickman's own beliefs in this.)
As near as one can get, it seems like the Good/Neutral/Evil gods in Krynn did a sort of detente where they have this "Doctrine of Balance" to prevent them warring etc. (no sign this in any way works, so it's very odd that it's supposedly a thing, they fight constantly through proxies), but that's just tyrannical gods acting in their own best interests, really, isn't it? There's no depth to it and no justification on the basis of "this is needed by the people of Krynn". Further, the very conceit of the Doctrine of Balance seems to be inherently Lawful, which means it isn't really arguable as TN concept.
If you go back to the
very earliest, proto-DL material--Jeff Grubb's homebrewed pantheon--you'll find the three pantheons are Lawful/Good, Lawful/Evil, and a loose Neutral and Chaotic alliance. And there's been a tendency for the past thirty years to emphasize the gods as forces of order unified against Chaos.
Even people who are insane experts on Krynnian lore can't square the circle on the Doctrine of Balance without inventing tons of stuff that simple hasn't been suggested to be the case (hell even with inventions and additions not in canon, it's pretty shakey):
Meditations on the Balance - Dragonlance Nexus.
Despite the date on the webpage, that essay actually dates back to 1998. It doesn't address one of the deeper issues--how can Evil be both fundamental to the world and yet be Evil as we understand it. This tension shows up from the beginning--the Queen of Darkness is described in
Dragons of Spring Dawning as one of the 'forgers of the world', and one of the three participants who will bring the world to completion at the Last Day, and yet in
Test of the Twins, we get Astinus--essentially the mouthpiece for Gilean--saying outright "Evil cannot create; it can only destroy."
Evil as something that must be opposed and should not be, but yet something that has a role in the unfolding of the world or divine providence, is something that can be worked with, but Dragonlance often gets away from that and the idea that "'tis not in mortals to command success" to the idea that "Evil is just as fundamental to the world as Good, and Good unchecked would be just as destructive as Evil."
I would argue that the Old Testament vibes here are merely superficial, underlying a weirder and more D&D-specific concept. Sure, it's a flashy-ass meteor and bazillions of totally innocent people are killed for "reasons" which vibes with a lot of 1700s and later conceptions of, for example, the Biblical Great Flood, but the core reasoning from the gods isn't the same - i.e. everyone is basically bad except for the few chosen people is the Biblical reasoning - indeed it's almost inverted! The Krynnic gods nuke the site from orbit to get rid of one guy who they feel is totally out-of-line, and it's not "the Good gods" alone who do it, either, it's the whole lot of them as far as we can tell - Good, Neutral, Evil together.
Looking at the two major inspirations/allegories for the Cataclysm--the Flood and the Downfall of Numenor--we
never get a sense that Istar has reached the levels of wickedness comparable to antediluvian human civilation or Late Second Age, imperialist, human-sacrificing, Melkor-worshipping, Numenor. One of several reasons the Cataclysm doesn't work for me, along with the religious allegories wrapped up in Istar and the post-Cataclysm 'abandonment of the gods.' (Note that for those reasons, I've never read the Kingpriest Trilogy, so maybe I've missed a reimagining of Istaran civilization.)
I think Hickman's original concept--made explicit in his proposal notes that he placed online a few years back, and still hinted at in places like DL12 and
Tales of the Lance--works better. The Cataclysm was simply the magical backlash from the Kingpriest's hubris-filled attempt to magically summon a god to Krynn to wipe out Evil.
There was also the hilarious retcon later-on that after the Cataclysm, it wasn't the gods weren't listening/were sulking, it was just that people didn't pray to them, which like, is quite clearly textually untrue in earlier Krynn works, given that people explicitly did, and that some of the gods basically apologise for sulking. But I imagine Hickman/Weis wanted to retcon that because it made the gods look real bad.
That's from the earliest material--the people only see the wrath of the gods and not the call to repentance. But the gods pull all their faithful clerics from the world beforehand, and apparently never seek to reach out to the people after that. (Yes, I'm familiar with the 'parable of the gem' from
Dragons of Autumn Twilight. Makes Krynn's gods sound like the idols criticized in Isaiah 44: "they see nothing, know nothing, and so they are put to shame.")
TLDR: Like the FR, Krynn's "Balance" thing is never properly explained or reasoned or details, it's just sort of mindlessly assumed. Again I trace the fault back to Gygax/Arneson for extending TN to include Good/Evil without providing any actual reasoning for doing so.
As mentioned, you'll find this whole 'balance' concept in a lot of fantasy from the 1980s--
The Dark Crystal, early
Masters of the Universe, I think
Legend--so I'm not sure where the ultimate source is.