[fwdude]

I’m talking about composition and construction. Comets obviously are constructed differently to “boil off” a tail, when other objects within similar distances to the sun do not.

[Robe]

Comets have been described as being “Dirty Snowballs”, generally believed to be made of ice and rocks.
Asteroids are largely minerals in nature, think Rocks, and or iron.
Asteroids are mostly in stable orbits unless disturbed by a passing planet, or even a comet on it's way in or out of the solar system