Casual is hard to define because in my experience I see several types of players: HardCore spenders, HardCore free, then I'd include Supported Players and then a big jump to 1 city retired.

I think it's because you can't eliminate (substantial) risk until you go down to one city and even then you might have to farm every few days. Unless you have cash to Holiday a lot.

So other than dedicated 24/7 peeps, the ones in the middle either have good alliances, friends or are immune to "blinking birdy syndrome".

Or another thought, new players can be casual until they either realize they can't invest the time/money to offset the potential loss of heros/cities or keep up with the "big" players. Or they decide to become big players and then have no life. Yeah it's interesting but a bit complicated.