it is because the Force at work here (gravity) is also dependent on the mass. Gravity acts on a body with mass m with F=mg. You will plug this in to F=ma and you get ma=mg. Cancel the m abd you will get a=g. This is true for all bodies no matter what the mass is. Since they are accelerated the same and start with the same initial conditions (at rest and dropped from a height h), they will hit the floor at the same time.