Not only timezones, but if there's a lot of people on one role-play, there can be pages and pages of posts before long and it's hard to keep up (even in small role plays). Now imagine one that everyone was participating in... Not the most desirable situation, eh?
Plus, not all members on this forum like to roleplay.
Now with those in mind, here's something else: it would be quite difficult to even try to organize everyone on here to roleplay.
It's a nice thought, good thinking, but with all the members we have, it just wouldn't work.
Sorry.