This question (or a similar one) comes up a lot.
There are a handful of insignificant sedevacantist fora, but none with even 1/10 the traffic or membership of CathInfo.
Let's just say there's a reason why CathInfo has so many sedevacantists. They want a board with some activity.
Especially for all the single sedevacantist guys (=plenty of time on one's hands), you're going to need a forum that's "CathInfo or greater" in terms of size and activity. Such a forum doesn't exist in a strict Sedevacantist flavor.
I personally have a theory (proven by a certain amount of scientific data) that people don't want to be a member of a forum where people are TOO narrowly focused. People want a bit of variety thrown in, or it's too boring. No one wants to be a member of "Rural Catholics of Irish heritage", even if that perfectly describes them. Variety is the spice of life. (Corollary: No variety, no spice.)
There's just no motivation to post if everyone on the forum agrees with you. Please don't take this the wrong way, but I'm quite confident about this point.
To return to my example, I can only read so many posts along the lines of
Yesterday, while walking to the family garden, I took in all the beauty of my acreage, and I sang "Whiskey in the Jar".
I'm listening to the Clancy Bros. right now. How about you?
The Incorruptibles was a spin-off of Fisheaters, set up to be "FishEaters without the drama". Today the forum no longer exists. About 120 people signed up, but they rarely posted.
Even Gladius_Veritatis and several other hard-core sedevacantists used to be members of
Fisheaters for crying out loud, and many might still be there today if they had a choice. Of course, when Fisheaters still allowed sedevacantists they only had around 700 members...
I personally think CathInfo is just the right balance of focus (Catholicism; no atheists, non-Catholics or de-facto heretics) with variety (sede, SSPX, and other trad Catholics).