Let's see...
I'm busy with RL stuff trying to keep myself busy so I won't have any unwanted drama popping into my life and trying to get a hold of my emotions.
Mayuri, Akon, Madara, and Reborn are all busy with work.
Raita has had a lot on her plate with appointments and group, also she's flooded with chores and has little social time lately.
Shura, Beranabus, and Mephisto have some RL problems to take care of and unfortunately won't be online much anymore at all. Family needs come first.
Amaymon is a lazy ass.
Bec is busy out looking for jobs.
No one really rp's with Emily besides Raita and I so she hardly comes online since no one else really talks to her.
Yylfordt...I have no clue. Ask Emily about him. Probably the same case.
Rin (If I remember right) is busy with classes.
Ten-Ten is having some family stuff if I remember...I saw her the other day but didn't ask. I know a lot of times though she can't get online because of her brothers using the computer.
I saw Kisho the other day too but didn't ask him why he hasn't been online much. I do know its his birthday week and he's been busy. I think he's been spending most his time with friends and keeping himself busy.
Shiro is busy in Japan and doesn't come on a ton since he has a crap load of responsibilities.
Bane I saw the other day. He doesn't get on much cause of his dad changing the password on the computer and whenever he manages to get on he usually has to sneak on.
Those are the only people I know about why they're busy. Everyone else, I have no clue why they don't come online as often.