They've lacked strong midfield presence for several years. Outside of Fàbregas, who have they had in the last 10-12 years with a high defensive workrate in midfield WITH skills on the ball?
They've sold more talent than they've brought in. For the last five years or so, this team simply wasn't good enough to sustain runs.
Look at the squad they put out against Bayern. Makes you wonder why Welbeck wasn't playing. At least he would provide a physical presence, even if he couldn't help in the interior third.
Weak midfield, can't pressure, can't tackle. When Ozil has more tackles than your defensive midfielder, you've got some pretty big issues. They're good enough to consistently beat bottom table teams and that keeps them 3-6, but outside of Sanchez there's absolutely nothing special about them. And Sanchez has baggage.