Probably something to do with you only winning four of your last 10 games, three of which were home games against Southampton (16th), Cardiff (19th) and Red Star Belgrade (who you just lost against), with the other being a win against Huddersfield (18th).
You're talking about how you "simply" need to beat Napoli, who are now level with PSG and have only lost one of their last 10, which was by one goal, away to Juventus, having won six of the other nine, and who have Red Star at home as one of their remaining games. You're also talking about how you "could" get some points at PSG, who have a 100% record in the league, have lost just once so far this season, and also have Red Star as one of their remaining games. With your current form, I wouldn't be overly optimistic (I'm certainly not about United's chances of getting through).