Off the top of my head, I'd say because
1) No external antenna
2) No SIRF 3 chipset
3) Not nearly as rugged.
Disclaimer: I have an original Vista (mono) and replaced it with the 60CSx just before the "H" versions of the eTrex came out. I have heard that the H chip is a vast improvement over the original, but still not as sensetive as the SIRF 3. Had I waited, I might have jumped on a Rino with an H chip, but I have been very pleased with the CSx.
Not everybody will necessarily appreciate or require the ability of the CSx to lock and hold a signal in very demanding terrain. I know that several here resent the GPS as making it "too easy" and lulling unprepared people into dangerous situations, but in areas where I travel off trail, much time is spendt under the forest canopy, and without a GPS, I would spend a lot of time with no point of reference.
-- FLIX