Sleepers by their nature have a very low occupancy per vehicle hence a high energy consumption per head. As yet we do not have surplus electricity.
Sleepers, very much so. Couchettes not so much. There are still quite a few seated coaches around in Europe with 6 seats per compartment in second.
(Didn't SNCF have some "couchettes" at one point where they crammed 8 people into a compartment with contoured "beds", and didn't charge a supplement? Not sure I'd fancy that though).
Also there must be a pretty large energy penalty for high speed trains, so running sleepers at "conventional" speeds must go at least some way to making up for the reduced capacity.
And obviously this doesn't work so well in todays multiple unit railway, but couchette coaches could be used for day travel enabling the rolling stock to be used during the day as well rather than just sitting around waiting for the evening.