Table top tends to be a more unknown and niche in any roleplay forum by my experience. I don't know the exact reasons but I can venture a few guesses:

1. People don't like being tied down to or having to learn a tabletop system. Even extremely simple ones can look overwhelming AND very restrictive at first glance, plus while the former goes away the later is harder to as many effects that people enjoy in freeform roleplay would just utterly break a tabletop system.

2. It's harder than it seems to find a tabletop roleplaying group for more niche systems, much the way finding them for a niche fandom is. However, for both those and more popular ones there is an even bigger hurdle, the need of a GM. GMs don't get characters in tabletops whereas its more common for them to have that kind of participation in freeform. As arguably the most essential person to have for a tabletop game, while at the same time there being an added responsibility far above that of freeform (as it adds managing and knowing the system very well on top of the responsibilities of freeform) AND the fact that for many, not having a character of one's own can make the whole scenario a lot less appealing... it becomes much harder to actually find groups, much less ones that are sustainable.

3. The reasons [@rapid reader] pointed out probably play a good part in it too.