Why time runs slow where gravity is high?
Gravity is cause by curved spacetime, so a gravitational field slows time.
Relativity shows that time passes slower if travelling at high speed on if you are in a gravitational field.
Einstein showed that gravity is indistinguishable from acceleration.
In a gravitational field, spacetime is curved. This causes time to pass slower. The stronger the gravity the slower time passes.
In the extreme, which is a black hole, time stops at its event horizon.