I'm puzzled by that, too.

They used to have C award seats last year (and probably more recently), but at the moment I can't see any on miles-and-more.

And the point is not in seating comfort (which is the same), but in connecting flights without Y award availability. At least with Fly Smart I don't see a way to book an award where one leg is lacking Y availability and the other leg lacking C.

Added:

I phoned with the M&M representative in Slovenia: Adria stopped opening C award space some year ago, for an unknown reason. If you book by phone you can request a seat on the waitlist and they will likely confirm it. If you book on the internet (Fly Smart) there is no chance, they say.