Maybe it's just a matter of UNC having better options this year than last year. UNC has a much more successful team this season than last season. Of course, they also have a different coach this season and a new coach may just prefer to give more minutes to another player. Walton's composite performance metrics (that are not dependent upon minutes played) have declined significantly this season (7.1 PER this year vs. 15.9 last year; .052 Win shares per 40 this season vs. .148 last season). That's not so unusual when a player gets demoted. I notice that a few other NC guards (Love, Davis, and Johnson) are playing noticeably more minutes this season than last season. Those players were the #13, #43, and #59 players respectively coming out of high school for their years.
UNC has 9 former top 100 high school players on their team and one who wasn't is a big impact 5th year transfer from Oklahoma. Must be nice.