It's not uncommon that airlines really restrict the number of FF seats available. I suspect they hope that they can get the miles to run out before poeple can use them. FF miles are a bit of a con, although it's been said that they represent an enormous financial liability to the airlines. But if they can't be used, they're as much use to the passenger as teats on a bull! Typical is the difficulty of using FF miles on AA in Business ona Sunday evening transtlantic from the US: too many possible paying passengers!
I suspect seats may become available at the last minute if they haven't previously sold them.