It's La Liga, there is not much logic here.
In Spain first teams and their B/reserve teams are considered almost like independent teams so clubs can't send down players registered for the first team to their B team, not even for a game or two to give players recovering from injuries some match fitness which is possible in all other big leagues AFAIK. I guess it's understandable because Segunda B (Primera RFEF) is not really a reserve league and teams like Barca B, RM Castilla and Villarreal B competing there mostly have much bigger resources than other teams in the division. It could be seen as an advantage for bigger clubs if we could send down likes of recovering Pedri or Ansu to play a game or two against Segunda B oppositions. Mostly just in home games though as I doubt we'd risk them playing on bad pitches in away games.
It's another thing when promoting players from B teams to the first teams. Most clubs wants (and need to due to financial restrictions) to develop their youngsters so La Liga is not preventing clubs from doing that and limiting their chances to promote from within. But yeah, I think there should be a way for clubs to send players to their B teams, especially younger ones (let's say U21) who don't play enough in the first team even if they're registered there.
That said, in summer we've had an option to register Yusuf as a Barca B player (which I don't think would be a problem for him to accept as he would surely prefer to play regularly there than warm first team bench) but for whatever reason we decided to register him for the first team instead (which I've said at the time was probably a bad move) and have now wasted (at least) 6 months of his development.
Similar story with Collado who was first promoted with a thought of going on loan but rejected that to stay and later we couldn't register him for Barca B anymore nor for the first team. The difference is he's a few years older than Demir and too good for 3rd tier football but probably not good enough for even our current first team. With all injuries he could've at least gotten a chance though but now we can't register him as a first team player again in January even if we'd want. So it's up to him to have a good loan spell at Granada and maybe convince Xavi to give him another chance next (pre)season but my guess is he'll be sold in summer.
