IMO the reason is because of 2 things. First the opposition are playing us differently nowadays. Last season teams tried to press us high and found out they can get done quickly if they did. So now they are sitting deep and countering or playing a medium block and pressing us when we cross the halfway line. Space is condensed and harder for us to find openings. We need wingers who are capable to break down teams with good ball control, trickery and passing. Thus Bernardo becoming an important winger for us.
The second reason is Pep is hellbent on keeping the ball. He doesn't like losing it since it leads to counter attacks. When the wingers are wide and balls are pinged to them, Sterling, Bernardo and even Mahrez are better at picking out of the sky and keeping it. There has been a fair share of Sane not being able to hold on to it. When it comes do being dynamic Sane is it for sure. However, even though he has pace to burn he does have longer strides and when trying to beat his man he can get dispossessed because of it. He's a little predictable and really left footed and teams will usher him inside and not let him get behind. He has to cut inside, but not a good right foot. Sterling is harder to predict on 1 v 1's. He has a lower center of gravity and good ball control and can go either way. A nightmare for defenders when you don't know which way they will go.
Plain and simple. Right now if everyone is fit, Sterling and Bernardo are our best wingers. They produce the most opportunities for shots on goal as a team. I love Leroy and hope he can work on a few of these things.