What makes an AGN active?
Need a supply of gas to feed to the black hole
(Black holes from 1 million to >1 billion solar masses!
Scales as a few percent of galaxy bulge mass.)

Collisions disturb regular orbits of stars and gas clouds
Could feed more gas to the central region
Galactic orbits were less organized as galaxies were forming,
also recall the “hierarchical” galaxy formation
Expect more gas to flow to central region when galaxies are young =>
Quasars (“quasar epoch” around z=2 to z=3)
Most galaxies may have massive black holes in them
They are just less active now because gas supply is less