I always remember Jose Baxter (not a great example) talking about playing for the reserves/u21s/u23s at an early age.
He said something like he got into the reserves at 15 and the standard was poor, it was either 20-22 year olds not good enough for the first team or pro's getting there fitness back that weren't interested. After a couple of years in the reserves you get bored and want to be playing proper football.
I also think with the amount of centre mids we have in our u23s in walsh, Davies, Williams, Baningeme all seemingly either level with him or ahead of him in progression it would make sence to let him go. Look at 29 year old Conor McAleny hanging around.
Also on another note, if the lad was leaving a bigger name club like liverpool, chelsea, utd etc, i bet a higher level of teams would be in for him instead of oxford n portsmouth. He is just as good if not better than rossiter and he got a move to rangers based on the club he came from