The thing is two factions have demo (allies), the other (axis) don't.
Axis blob is always in danger of blowing up with demo, but the allies blob is never in such danger and they just retreat as soon as they see a threat.
You can see 10 out of the 9 videos
on coh2.org trending is about the laugh after blowing up an axis blob with an allies demo. that tells something.
Simple as that.
Specially since in majority of the games you see blobing armies being used by both axis and allies players.