The reason can be very simple. Some ppl prefer easy game rules and somehow simple games have this "mysterious" pull such that one wants to play again and again.
Tetris, Rubik cube are still popular nowadays so there must be something that attract correct?
I shall leave it to some PhD thesis to answer this "funny" attraction :P
In recent times, big game makers also notice this trend of "casual gamers" It can be a sizeable market. They must be wondering, simple games that require few resources investment can draw in crowds? They should engage some PhD holders to do research on human brains. What trigger the human brain to play game. Does it have to be action-packed filled with action, gore, sounds or just simple maths logic can achieve the same effect too.
I would say I am also a casual gamer most of the time. Sometime ago I used to play AOE2 for hours at a stretch and NFS till 2 months ago, but along with daily jobs, these games get too time consuming...
The casual games are sometimes fun too. And sometimes hard too.
Last week, I was trying to shoot 4 stick-figures popping up from behind a tree, which just seemed impossible till my drunk room-mate came and shot all 4 in one go... and then took the mouse away !!! *&^#^%$^%$^
For break, I sometimes play simple online games or a short ride in NFS.
I agree with chrisname. I've been having a lot of casual fun with Just Cause 2 (very recommended if you like to blow up stuff). On the other hand, when I play an RPG like Baldur's Gate it's never casual.
Oddly enough, Super Mario Galaxy is Nintendo going for their hardcore market. People often assume hardcore must be RPGs or shooters while Nintendo has held a hardcore audience for years with first party titles like Mario, Kirby, Donkey Kong, and Zelda. Nintendo is a different bread then most, and although their console is blatantly soaked with shovel-ware, the hardcore fans are the one's playing Mario.
Nintendo relies on nostalgia too much at times however...