Too expensive for a big enough SSD drive. Developers probably don't want to see there margins shrink either due to added memory card costs. Even if most of the cost goes to the customer, it is still money going nowhere.

Although there are other 'advantages' to card based delivery. Sticking to a hdd only machine with SD cards as delivery you can lock out the 2nd hand market by writing the machine ID to the card when you load the game for the first time. You'll still need to install the whole game anyway since cheap enough SD cards won't be fast enough to play from. Install once and you can only re-install it to the machine you first installed it on.
Seems a frighteningly good intermediary solution on the way to going full direct download with no real ownership.

I think they'll stick with high-capacity discs. It should still be the cheapest way to store retail games (short of exclusive digital distribution). However, I could still see them going with ssd. SD card-based carts have been working out pretty well with Nintendo's last two handhelds, so it wouldn't be a stretch to see it adapted for a console.

You know if they had all the games on high speed solid state that would mitigate limited RAM as they could stream in content fast enough you wouldn't have to cache as much.