A number of factors really. I could go on about injuries to key players at the wrong time, the new PL fashion of lower table teams parking the bus, playing away from home all season, and the top 6 aside from City being about equal all taking points off each other. Yada yada etc.
But I think we've played better in CL as we've been selling out Wembley. There's a better atmosphere with a full crowd. Like we had at White Hart Lane, and will do again next year. We'll get 85k - 90k for Juventus, but who wants to pay Wembley ticket prices to see Stoke or Swansea put 10 men behind the ball. And even though a 40k attendance isn't low by most standards, that's not even half-full. Like an exhibition match, and our team has played like it's an exhibition in some of those games.
We're not in 5th because we've lost the odd game to another top six team, we're there through draws with Swansea, Burnley and West Ham etc. Can only hope our big-game Charlies (e.g. Alli) are more motivated by Italy's best in front of a full house. Kane will turn it on whoever we play, but some of the other younger players aren't quite at that level of commitment (then again, most aren't).