Lubov doesn't do anything active to save the cherry orchard; she wrings her hands and waits. But she's the emotional center of the play. We feel her joy at the top of Act 1 and empathize with her grief at the end of Act 2.
Lopakhin, on the other hand, is very active in the play. He's a source of energy in each act, and instigates the major plot upheaval, the sale of the orchard. If we believe the play is a comedy, Lopakhin is the hero. In dissolving the orchard, site of his past enslavement, he secures a happy ending.