Question

why does the blackhole have maximum amount of gravitational pull

Answer 1

The primary formation process for black holes is expected to be the gravitational collapse of heavy objects such as stars. Gravitational collapse occurs when an object's internal pressure is not sufficient to resist the object's own gravity. This happens in stars when they run out of fuel to maintain their life cycle. 

This is not the only process which creates black holes. They can even be formed in high energy collisions that achieve sufficient density. Another possibility is for a black hole to merge with other black holes to produce supermassive black holes.