The Roman Empire fell due to lazy writing.

The guys writing humanity's story arc hit a dead end with Rome, and had to think up of a way to kill them off so that we could do the "Medieval Post-Apocalypse" plotline.

You ever notice how the Huns showed up out of nowhere around 370 CE, were basically built up as a HUGE threat for Rome, and died alongside Rome in the 450's, only for their empire to basically disappear immediately afterwards? It's because the writers needed a way to kill off Rome that wouldn't actually stick around afterwards. Sort of like how DC Comics created Doomsday out of nowhere to kill Superman before immediately killing him in their big fight.

I can't give the writers TOO much flak here, since they did a great job with the Medieval era arcs, but the way they killed off the Romans was pretty lazy.

Rome collapsed because they were releasing too many marvel movies, i knew it!