Could be as simple as people don't see the need yet for the speed. I have friends on cable 10meg who could easily have 30 or 50 but really don't see any reason. They're not IT types
I am an IT type, and a fairly heavy user too, but the ~12Mbit I was getting downstream from Be was fine enough. The 7ish mbit I got on an earlier ISP was fine too. Fast enough for anything interactive, and if I was downloading anything big it got done overnight (or otherwise unattended) so I didn't notice the delay. The near 40Mbit I get now is very nice and I make good use of it, but I could easily live without it.
The big draw for me was the much better upstream speed (from ~1.5mbit on ADSL2+/AnnexM to near 10mbit), but that doesn't really affect most home users. I run a couple of small servers (well, one server with a couple of virtual servers running on it) and sometimes have reason for people to pick up large files from me - but that isn't going to affect most home users much. The biggest difference that could be advertised by the ISPs would perhaps be uploading a hiqh quality video to youtube or facebook. Of course the many people might make use of the better upstream to help keep share ratios high on private torrent trackers, but that isn't something that ISPs can advertise as they won't want to be seen to be encouraging breaking the law.
It isn't just that people do see the need for the speed, for many there simply is no real need for the speed. They'll take it if it is free (in terms of both cost and hassle) but it is a luxury which they don't want to pay for (or go through the hassle of switching for).
Current ISP: Andrews & Arnold (AAISP) via FTTC at ~40Mbit down (has occasionally dropped to ~36) & ~10Mbit up, joined July 2011.
Previous setup: Be Pro with UploadPlus (ADSL2+, AnnexM), 12ish Mbit down, 1.6 up, happy customer for ~2.5 years.