Likely because you have not logged into these forums in the last four weeks.. you must at least log in every 30 days here to be considered an 'active' member.
If you are already logging in to the forums once every 31 days and are still getting suspended for inactivity, you may be interested in reviewing a page I have on my website to make sure you are doing everything correctly:
If you are unable to log in to the forums at least once every 31 days, you can pay a mere $3 to not have to log in for an entire year. More details on that on the above page if you scroll up.