Question

Why does a ball accelerate as it rolls down a hill?

Answer 1

The ball accelerates because of gravity.

Answer 2

Because there is a 'net force' on it.  That means that all the different
forces on it do NOT all cancel out.  Whenever there is a net force on
an object ... not zero force ... the object accelerates.  (That means that
either its speed or its direction changes.)  We've known this for about
300 years now.
The main force on the ball is the force of gravity.