I see so many deleted accounts and sometimes wonder why those accounts would have been deleted. Are moderators allowed to tell me why a user's account was deleted? For example: " this account was deleted for the violation of rules." How can I do that if possible?
No, we can't. This is not an official answer but it pretty much sums it up:
Why the user was removed:
This is between the user and the mods or SE, such moderation matters are considered private and are not made public, unless the user chooses to do so himself.
Note that it's also possible that the user requested account deletion themselves.
You might want to look here as well: Are user suspension and user deletion discussions on-topic for per-site meta?