I think it has been gone over before with a number of players (Wilson and Lawrence mainly) but I don't think it is any slight on Thorpe's perceived abilities as a centre back, more that we needed to leave some players back to cover our other pre-season commitments. I reckon Thorpe will get a few run-outs should draws be favourable in the cups this year.
Wilson was going on tour anyway, but he got injured. Lawrence is a year younger plus had lots of competition in his positions so both of those are understandable. Also both have already made their first team debut in the Prem as well.
Thorpe is 21 and hasn't even played a game or had a sub appearance for our first team yet and never gone on pre season tours. It's a bit odd as we were short in numbers at CB as well this time. (He was away with England U20's last time IIRC.)
He has always been among the top performers at all age levels for us and was even the standout defender for our youth cup winning crop, but Guess he was bit unlucky with getting a good loan so late in his development and then getting injured so soon into his loan. Plus he hasn't had a growth spurt that Keane had so his height and leap have remained a bit on the shorter side for CB.
It's really make it or break it time for his United career this coming season, i imagine and i don't think actually staying in the U21s with few cup games will do him any good anymore. He pretty much bossed it more than Keane as well for the first half of last season before both went on loan. He now needs to show he can actually do it in first team games somewhere regularly, and is tough enough to do so against Grown men consistently as he showed somewhat in his brief interrupted spell at Birmingham from reports.
Edit - Interesting, btw how Joyce singled him out again along with Harrop for praise vs Barnsley mentioning how he and the defence had dealt with their big guys specifically.