Yes, it might be known why Komala can't learn rest since it was born sleeping meaning it already USED Rest, but still, if it is born sleeping why, can it learn Rest?
Komala's ability Comatose means that the Pokémon acts as if it were already asleep, despite still being able to attack on any turn. Rest is absent from its movepool because you cannot put a sleeping Pokémon to sleep. While there are some benefits of this ability, particularly with status moves, there are also drawbacks. The move Dream Eater will always deal damage, and I believe Wake-Up Slap always hits at double power. Rest-Talk is off limits for this Pokémon.
It's one of those things we may never know :D It's like how some bird Pokemon can't learn peck and how Gligar, Gliscor, and Garchomp can't learn Fly while in the anime, they are seen, flying :D