A person can have the same *motivations* as a cleric or paladin, and have different *gifts*.
If someone wants to fight the cult of Tiamat, and lacks the WIS for Cleric and the STR/DEX for Paladin, but has lots of CHA, then Bahamut can either grant that person a pact, create a level 1 warlock, and gain one more warrior against Tiamat; or Bahamut can decline, leaving that person at 0 level... and not gain one more warrior against Tiamat.
Which is the more responsible choice, from Bahamut's perspective?
There are no direct Paladin powers/abilities, in 5e, which depend on Dex or Str. All of the actual magic they wield is dependent on Charisma already--and, for at least one encounter per short rest, a Devotion Pally (the Oath which makes the most sense for Bahamut) can make up for not having good Str (or Dex) with her Charisma.
It's not just a matter of having greater forces, which I tried to communicate earlier but I guess I didn't. Bahamut is a Lawful Good guy. I believe he prioritizes Good first, but Lawful is always a consideration. The fundamental nature of the Warlock pact,
in my opinion, doesn't jive with that. While it might be a "binding contract," and so "lawful" in that sense, the implicit nature of the "Warlock" class is that you can do
whatever the hell you want as long as it doesn't violate the (implicitly) narrow range of unacceptable behaviors. I think Bahamut is substantially more selective than that. He absolutely appreciates and welcomes anyone--of any class--who wishes to fight the Cult of Tiamat, and for really helpful people, he might even try to float the occasional divine intervention as a "thank you." But that's a far cry from handing out power to just Joe Anybody who happens to dislike Tiamat.
So...I genuinely believe the more responsible choice, from his perspective, is not to give out power to absolutely everyone who meets the bare minimum requirements of agreeing with one narrowly specific goal. Power is easily abused. Giving out power to anyone that you think
might use it to further your ends is an abuse of power itself. Bahamut, being a god of protection, hope, and justice, would probably not be very inclined to make "vigilante" type people unless he felt there was a dire injustice that needed to be corrected sooner rather than later.
Also, remember that I'm coming into this from 4e, where even to become a Divine class proper, you have to have Investiture: a ceremony whereby a fragment of divine power is bestowed upon you; generally done by the clergy, though it requires the deity's permission and
can be done without the clergy if the deity wants it badly enough (the implication is that non-ceremonial Investiture is more "expensive" due to the limitations on overt Divine intervention in PoLand). It's very, very explicitly fluffed as something the gods take with utmost seriousness
because they can't take it back. Powers that are constantly held on the end of a "gotcha" string are stupid and encourage DM abuse merely by existing, so I'm never going to consider a "Bahamut pact" one that Bahamut can just yoink if he feels it's not working out. (And that would strike me as a not-particularly-lawful action anyway).