Mass-production vs one-off customization. Even with robots and CAM to handle the grinding, it still takes more time and effort to grind two lens surfaces instead of just one... and more effort to calculate custom lenses instead of blindly grinding another standardized design on the other side.
That said, I think that competition from cheap online labs will force traditional retailers to reduce the cost of freeform lenses and market them more aggressively. If you can take your prescription and buy a mediocre pair of glasses for $15 online, why would you pay $50-100 for an equally-mediocre pair? On the other hand, if a mediocre pair is $15, but a freeform pair is $200, and the freeform pair is sufficiently better to feel like a HUGE improvement, people will pay the higher cost for the quality pair (even IF they buy a cheap mediocre pair online to keep as a spare pair). By (US) law, your "prescription" (sph, cyl, axis, prism) have to be provided on demand so you can use it elsewhere, but the measurements needed to make custom lenses for a pair of frames are still considered proprietary (and are pretty damn hard to measure without the proper equipment), so they aren't required to share them with you. Ten years from now, mediocre online glasses will be regarded the way drugstore reading glasses are now... something that's better than nothing if you're poor or need a spare, but blatantly inferior to "real" glasses.
As far as SV lenses for astigmatism go, the main advantage of two-sided freeform fully-custom over single-sided freeform semi-custom is aesthetics... with control over BOTH surfaces, you can neutralize things like magnification/minification (so your eyes don't look larger or smaller to observers seeing them through the lens), and adjust the base curve near the periphery to allow thinner lenses. If you only have full freedom over one surface, you can optimize for optics at the expense of thicker lenses, or thinner lenses at the cost of optic fidelity, and your ability to neutralize out magnification/minification will be severely constrained.
From what I've been read, single-surface semi-custom freeform is a HUGE step forward from single-surface lenses ground with traditional standard curves, but the difference between single- and double-surface freeform (for SV astigmatism) is more like the difference between 480p24 from a DVD and 720p24 from Blu-Ray... it's there, but it's not nearly as dramatic as the difference between nominal-480i60 from VHS and 480p24 from DVD.
I believe lenses marketed as "aspheric" are basically designed the way freeform lenses are (with raytracing), but are mass-produced in only a few permutations of sph+cyl... and usually, optimized for thinness over optics. Personally, I wouldn't bother with them... they cost almost as much as single-surface freeform lenses, and don't provide nearly as much optical improvement.