It is mainly because we kept playing on our right side. If we have a proper left winger, Gibbs and Monreal would have had good chances to cross too. A properly overlapping fullback would have created more dangerous chances than crosses, because he can just cut it back for a tap-in. Debuchy, Coleman, Zabaleta does it well.
That said, it is also a fact Sagna's crosses this season have been quality. Giroud should have scored from a few of his crosses.