Ok...this may be a dumb question but I was just wondering if there is a simple reason why planets are mainly round...while asteroids are iregular in shape.

The planets are large enough so that thier own gravity is enough to pull them into a sphear. Much like a supermassive star's gravity can crunch it into a neutron star.

being Round/Spheroid is the most economical shape with respect to a central gravitational pull, it`s ideal as there`s nothing left to "fall".

 

Asteroids, although they Do have a gravitational field, it`s so weak (low Mass)as to have little effect upon the materials they`re composed off.