Free: Quantity over Quality
Casual: Quality over Quantity 
Advanced: Quality and Quantity 

I believe some people just find it hard to write more than a few posts of good quality in the time that they are given. Everyone has different writing and reading speeds and trying to dedicate time for that can be difficult for people's busy schedule. I can see why casual is popular because 2-3 short posts of can suffice if you're held up for a while and it can keep the ball rolling.