Shuma-Gorath may be similar to Lovecraftian fiction and all its ancient horrors, but Knull would feel right at home in a story of eldritch horrors and elder gods, because he is both of those things.
Knull is a dark god, born before time even began in this universe. At first, he was pretty much happy to just silently and contently drift through the endless abyss he called home. Until his !*$% neighbors - the Celestials - started creating the universe we now inhabit. Knull responded in pretty much the only way he could have: by forging a sword out of his first shadow and taking one of their damn heads off. He now seeks to do the same to the rest of us until there isn't a single shred of light left in the universe.
Now, this guy may sound like the perfect setup for an MCU big bad. But the deeper you go into Knull's mythology, the more you realize that this guy would have no place in the more general audience friendly MCU. Kinda like the rest of Jason Aaron's work that isn't Lady Thor, to be honest.
Sure it would be cool to see how the Symbiotes came to be, but frankly, we don't really need this level of deep lore. Especially when it's for a character this utterly disturbing.