Probably because:

1. It's still not really affordable compared to renting.
2. Shared ownership is a complete con, what's the point in owning half a house - you might as well just rent.
3. They can rack up the service charges and rent on the 50% you don't own and there's nothing you can do about it.
4. There is little risk of house prices rising any further from their current already ridiculously elevated levels, so there is no rush to buy, and if you wait a year or so prices might even come down!