I think it is because the manager of the national team picks who becomes the U21 boss, the same way how you can offer coaching roles to staff. I applied for the England job on my latest file after Capello retired 2 years into the game, however the FA appointed the U21s manager Stuart Pearce instead. I then decided to apply for the vacant U21, however also found that I couldn't. A few days later it appeared on my news feed that Pearce had appointed Gareth Southgate to manage the U21's (I would have been the much better option!), as I was subscribed to all news that happened with the national team. Whether you actually can be appointed as U21 manager or not I'm not sure, but it would be awesome to be able to blood the next batch of players through, and then possibly become national manager and bring them through further.