Because Nani has proved in the past that after few games under his belt he usually ends up in good form, and that he is usually poor in first game our two he gets a chance(he even showed that this season). With Young, his form is usually shit whenever we play him, but he can be good at random times. He comes in after few months of being injured and plays good game against City even though it's his first after the injury but give him run of few easy games and he plays like shit. Next time against City he plays like garbage, but in next easy game he isn't bad. Totally random.
Plus he has showed nothing to suggest he should be our first choice winger, or for any other top team. He has proved many times that he isn't good enough for Manchester United(including this season too), and I really don't see any reason for playing him at all when our other options are fit. Same goes with Valencia for me. I haven't seen player who got more chances and who disappointed more than Valencia(actually I have bigger problem with Valencia than Young because he is one of our most featured player in last two seasons and attacking wise he gave us nothing in those two seasons and he got so many chances). You just know with those two that they aren't good enough to be our first choice anymore, they have proved that, but with Nani there are still question marks if he can be good as he was. When he gets at least half of the game time Valencia gets and if he disappoints again I'll be first to admit that he isn't any better than Valencia or Young anymore and that is time to move on, but instead of ending up with 3 shit wingers why don't we actually try to give him a chance, what's there to lose? In worst case we get shit winger playing for us couple of games(big deal, we had that before anyway). Best case, we get good winger again and instead of having 3 shit wingers, we have 1 good winger and Young and Valencia as backups.
Even if you don't like Nani, I really can't see nothing bad in playing him ahead of Valencia and Young.