It is due to the history of the forums.
First there was the Olympus Talk Forum, and all was good (well mostly, there were some disagreements such as my divisive post to the Flower Art challenge, that in hindsight I wish I had just skipped).
When the E-1 came out, there was some tension between the traditional users of the Olympus Talk Forum with fixed lens cameras, and the shiny new DSLRs. So they created the Olympus SLR Talk forum (note there were 2 Panasonic 4/3rds DSLRs announced along the way, the L1/L2 which didn't quite fit in the name).
Then when Panasonic and later Olympus announced the micro 4/3rds cameras (G1 and E-P1 respectively) there was a lot of tension between the two systems. Given how Panasonic makes a lot of micro 4/3rds bodies and lenses, as well as Olympus (and now OM instead of Olympus), it is useful that the m4/3rds forum did not specify a brand name.
While I have posted to all 3 forums, the Olympus SLR forum at times has struck me (rightly or wrongly) as having a larger population of people who don't want to deal with newbie issues on most posts except for the picture posts and challenges which came from the Olympus Talk Forum. Sort of the stereotypical old guy putting up a kids get off of my lawn sign.
With the current grouping, Panasonic in particular is split different ways. You have the Panasonic fixed lens cameras grouped in with the micro 4/3rds stuff. I don't recall if the L-mount stuff is a different forum, but if it isn't it should be.
Now of course OM has stopped making fixed lens cameras except for the TG6, which they crank out, but it is unclear if there will ever be a TG-7. Similarly, Panasonic is still selling their last generation fixed lens cameras, but it seems clear there will be no new cameras. Thus both forums are probably filled with the old guard faithful, but it is likely not a growth area.