i wondered about this also, till i had a friend with a 6 lb baby who couldn't get the straps tight enough in her front adjust,a smaller baby could get a tighter harness with the back adjust... ( in her case the straps where at baby's shoulders but harness wouldn't tighten enough!)
Honestly, I think that if they were still making it just so that they had a seat to accomodate itty bitties, they's make one or two versions of it - not make
mainly rear-adjust versions. I'm sure they do it for the same reason businesses do most things - money. I assume it must be cheaper to continue using the same mold(s) for the back-adjust seats than to create (and get certified) a new mold, the seat still sells like hotcakes, etc.
I also bet they probably don't get that many complaints about them - most people don't use their seats correctly, including tightening the straps adequately, so they don't realize how hard it is to do correctly with the back-adjust seats.
(I was just thinking "why do they still make these" last night as I was browsing online for infant seats - there are a few cute patterns I like, but they're all back-adjust! Grrr!)