It was a messed up season to begin with for rookies because of Covid - but since he played last year as a 16-year-old, even if it was for only 21 games, he still played the majority of the season he was eligible for so he wasn't counted as a true rookie. whereas Yager, shouldn't have played last year but covid restrictions made it so he could play a fair amount of games as a 15-year-old, even though in a normal year he wouldn't have been able to. so he was eligible.
If we're being completely honest though, Bedard deserved the ROTY this year but there were so many things messed up this year they probably said "Rookie of the year goes to a 16-year-old not named Bedard"