I think there are several factors though, each decisions are individual. For example, McNair hasn't been that impressive of late, so therefore simply wasn't picked. He has been picked before though, quite a few times, so Van Gaal obviously knows what he can and can't do, and then now reserves the right to pick him or not. He's very much part of the first team squad, and started the first leg anyway.
Last season, Love and Riley were all a year younger, and probably not ready. Van Gaal came in and Young impressed at left-back from the tour, and had thus proven he can play there for Van Gaal. Blackett was given a chance to stake a claim at left-back, and proved himself rubbish, so Young naturally got to play. This season, logically, Young would get some games at left-back when needed, he had proven himself to be a good left-back from the beginning. Same as Valencia, who had played right-back for a while, way before Love would have been a consideration. These lads were likely not ready at first, and the senior players did well, so were ahead. McNair (who Van Gaal called our right-back for the next 10 years) was played ahead of Valencia often enough, until he then either got injured or simply showed himself to not be very good.
With Wilson, he can have no arguments about the amount of first-team football he's played under Van Gaal. He played plenty of football last season, including at centre-forward from the start against Liverpool at Old Trafford in the League - the exact same tie Fellaini was chosen to play ahead of him this season. What he should be more annoyed about is the fact he has been largely rubbish when played. This probably had a greater bearing in him not playing against Liverpool than his age.