It is a mystery. Why do black holes grow only to a size? For this, astrophysicists at a university in Pittsburgh (USA) have used simulation.
The simulation has been based on the quasar, which is seen as a bright area of great mass, with a black hole in the center and devouring the surrounding stars. For it seems that sometimes quasars attract more gas than they can ingest; part of this gas is absorbed, but the rest is jetted. And over time this process kills the quasar, stopping black holes from growing.
For simulation they have used a supercomputer. And they have taken as base two galaxies that are being touched (in the image is given the sequence of the process from top to bottom). The black holes of these galaxies come together forming a huge black hole. The system, the quasar, emits 0 gas into space. One day it stops growing and the couple disperses.