Arguably there was a lot more meat to it than the horus heresy - it covers most of Vandires lifetime and all the events set in motion by him across that time.
The build up in M35 is also fairly substantive with Nova Terra, the Moirae Schism(which continues throughout M35-36), and the Cataclysm of Souls all setting the scene.
The thing with HH
was that there was nothing there, the writers essentially had a blank slate with only two points of reference, Horus turning traitor and the Siege of Terra. They could make up characters, draw lines to things we knew from 40k
and explore the mythical Primarchs and things like how the Adepta came to be.
They could deffo explore a lot of things for the Age of Apostasy but IMO
the problem is that there is so much we already know about it. The War of the Beast and HH
were events that happened but had little to no story which meant writers could do a lot with little restriction. I see a lot of people say "just leave things as mysteries" with regard to things like the Clone Wars from Star Wars and the HH
but there's no denying both of these events were important to their universes and expanding on them gave people more ways to play/tell stories/enjoy a setting. I think with the Dawn of Fire and most new books being set in the Era Indomitus we'll be hard-pressed for "historical" 40k
any time soon.