We all know Luca was a devoted follower of Don Vito. My guess is that it was Vito's fear of losing the friendship of Luca. You see Vito was very careful with his friendship with Luca. He might be afraid to screw up some petty thing when meeting with Luca, which would reduce Luca's loyalty towards himself. Luca didn't have a normal mind, making him somewhat uncontrollable. We all have such kind of person around ourselves.