Comets & asteroids are not classified as planets because, to be classified as a planet, they need to do three things.
The International Astronomical Union, which decides such things so that we all have the same definition for things and so don't confuse each other by using the same word for different things, laid down the 3 rules for planets back in 2006.
1. is in orbit around the Sun.
Comets & asteroids score on this one. They are in orbit around the sun.
2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape)
Comets & asteroids lose here. They, particularly asteroids, are not round, and if they are round, comets here, are't round because of having sufficent mass.
3. has "cleared the neighbourhood" around its orbit.
Comets & asteroids also lose here. There are lots of comets & asteroids in very similar orbits.
Pluto was kicked off the list of planets because it failed No. 3.
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