The main issue with black holes is that you can't feed them too much stuff at once unless it falls directly into them, which won't happen in a galactic environment since everything has orbital momentum.

Once a black hole has an accretion disk, the disk itself has so much energy that it will shove away extra matter trying to fall into it.

Thus, one theory on the formation of supermassive black holes is the "black hole star".

Put simply, the idea is that in the early universe in areas where there weren't any particularly big things or galaxies it would be possible for light years worth of diffuse gas to begin accelerating towards the same area, which could collapse directly into a singularity.

It would still have enormous amounts of gas falling towards it, though, and the sheer gravity of all this gas could overcome the energy emitted by the relativistic accretion disk and continue to grow the black hole at a prodigious rate.

Eventually the balance would break and it would explode, but most of the remaining gas might not reach escape velocity, this would be the "seed" of a potential galaxy.