It's a combination of a mentality issue and also having lesser quality than them. The two redshite pundits each mentioned these two things. They have always had much better players than us, plus being a derby they give 100% effort (as opposed to games they take for granted against the likes of Swansea, Burnley etc.), therefore effort alone won't beat them as their quality will overcome ours. Essentially we are relying on luck (our best chance was perhaps when Gerrard got sent off a few years back and they beat us with 10 players...even then we had Phil Neville in midfield against Xabi Alonso..so that cancels out the extra man we had).
I think if we had Schneiderlin today it would have been a different game...we missed him so badly as Barkley is too wreckless and lacks a football brain while Davies is just a kid. The RS were at their weakest with that turd Lucas in there with Henderson and Lallana missing.
Someone touched on it but look at the forwards they have had over the years in the winless run...Owen, Torres, Suarez, Sturridge, Coutinho, Mane. They have destroyed us with their pace, movement and quality. They have had an endless stream of top attacking players, we have had the ilk of Osman, Beattie, Bent and co. Lukaku is easily the best forward we've had in the PL era, but he can be nullified as he is heavily reliant on service...he's not going to pull defenders all over the place. He's static, slow on the turn, weak first touch...he can't produce on his own. He's quiet for long spells against better quality opposition. If he's not scoring he does nothing...he's almost the opposite to big Dunc in this respect as the big man wasn't the most prolific of scorers but he was always a presence.
The players do get it, the effort is there, unfortunately the quality isn't.
