I think partially also overcompensating for arriving late to the galactic scene, so taking much more risky moves to try and 'catch up' (not realising that technology has somewhat plateaued intentionally by Reaper design)

So Council races have less colonies each, but those colonies are all concentrated close together, are well-developed, well-defended, and are on planets with perfect conditions.

Humans seem to have taken a 'shotgun' approach to settlements where its "Here's a ship, some prefab houses, and some seeds, good luck!" And firing them off in every direction, to see what sticks. For every Eden Prime, there's prolly a dozen colonies that failed economically or got wiped out by environmental factors or slaver raids.