The level of coma and other factors are related to how close the comet comes to the sun. Oumuamua came extremely close (about 25% the distance from Earth to the

The level of coma and other factors are related to how close the comet comes to the sun. Oumuamua came extremely close (about 25% the distance from Earth to the Sun), and displayed no such characteristics. This is why thought it was originally classified as a comet, it was quickly reclassified as an asteroid. But then it started accelerating which means it's also probably not an asteroid. And on top of this all a comet would also likely have torn itself to pieces accelerating given the shape and tumble of Oumuamua. It somewhat cleanly refutes every possible classification leaving only the ¯\_(ツ)_/¯ group of celestial bodies.