Blame ADA. If you want to compete in IAPLC you can't post your tanks anywhere public for feedback, critique, etc... So "serious" aquascapers have private groups to discuss aquascaping.
There is also significant overlap with the tank journals forum-- even if you are doing an "aquascape" it might as well be in the tank journals section as it's easier to keep track of what you've done and provide context for the tank you're talking about for other users.
Finally, there just isn't much to talk about with regards to aquascaping. Sure, I could write an essay about trends in diorama-style aquascapes both towards and away from natural scenes and landscapes, but there might be only two or three people on here who would actually care, or who follow aquascaping closely enough to have anything to contribute. Add on to that the fact that I can't link IAPLC pictures and everything is a whole lot harder.