The best answer is simply: That's the way gravity works.
If you take Newton's deceptively simple formula for the gravitational force
between two masses, and if you have enough geometry and calculus to
massage the formula around, look at it from different angles, and follow it
through to some of its implications, then some wonderful things happen.
The first result is that all of Kepler's laws of planetary motion fall out on the
table, and you no longer have to ask "Why do the planets do that ?" They
do everything they do because that's the way gravity works.
You also discover that neither the size of the orbit nor the speed in orbit depends
on the mass of the orbiting body. The speed depends only on the size of the orbit.
You're happy about this because it means that an astronaut can do a "space walk" ...
separating himself from his spacecraft but still staying in the same Earth orbit that
the spacecraft is in, so that he doesn't go sailing away from the spacecraft just
because he has less mass.
And you also discover the strange fact that the larger the orbit, the slower the
orbiting body moves. An astronaut on a space walk in low-earth-orbit is orbiting
faster than the moon is!
It's just the way gravity works.