That probably factors into their thinking a bit.
I'd also bet most people (not necessarily most enthusiasts mind you) would be quite satisfied with solid bass reproduction down to 25Hz or so, given that most movie theaters aren't delivering anything better. It's relatively few people that have interest in plumbing the depths.
Finally, I'd also toss in size as being a factor. I mean, regardless of whether you're talking sealed or ported, relatively speaking, you're going to need a lot of subwoofer to competently dig into the 20Hz and below region. And lets face it, for most people, that's not exactly a plus.