I think I can think of a few scenarios:

-They don't accept the diagnosis

-They are not on meds and may be manic and off on a path that doesn't include internet contact

-They are depressed and have trouble doing much of anything

-They don't like being online

-They worry people will find out who they are

-They don't know about online support or how helpful it can be

-They think they can do it on their own

-They are using and not in the state of mind

-They are paranoid

Just some ideas off the top of my head. I wish there were more people online, it makes the illness much more bearable when you know you aren't alone in the struggles I agree.