Here in my immediate area (Long Beach-Seal Beach in California), I have seen a few logs by others but I seem to be the only one really into it.
My personal belief on the smaller number of BM Hunters vs. Geocachers is the payoff for finding a BM.
When I explain Geocaching to people the response is usually an interested "What do you find in the caches?"
When I explain BM Hunting, the repsonse is "What do you do when you find the disk? Nothing?!?!?" followed by a 'whatever floats your boat' look. The response is a little better if I mention that I am helping to maintain a national database for the marks.
BM Hunting has a more mental payoff than a tangible one so is less appealing.
It is also a bit more uncertain as the benchmark may or may not be there. You can look on the geocaching site and see that someone found/didn't find the cache last week whereas with a BM, it may not have been logged in the past 30 years (or longer). Even the most recent loggings for a BM may be five years old. I don't think I have seen one that was logged with the NGS after 1998.
There is also a research element that may be involved if the area has changed. I personally enjoy researching what happened to a landmark described in a Station Description.
In summary, I think that most BM Hunters will remain lone wolves.