Understand why many feel that international roles can be combined with other roles, as they're naturally episodic in nature, but why would the U21s role in particular be part time more than say the senior manager?
Now that games have been shunted together so that they often play two games in 5 days it that has reduced the amount of actual time spent with the players. There is i would say a smaller pool of players to watch than the full team (although they may be spread over more teams).
Also as the under 21's manager you are less likely to be invited to fly over to Hong Kong to give talks for huge amounts of cash!!