Luigi is often depicted as cowardly and somewhat of a loser at times that why would Waluigi want his shoes? Does that mean Waluigi wishes Mario regards him as a brother? Which would explain why he would hang around Wario if that desire is somewhat met with Wario.
The official reason is unknown but in Smash Bros trophies Wario is describing as follows :
"Waluigi reportedly spent a lot of time training by honing his ability to antagonize the Mario brothers."
Among Mario brothers, Luigi is his most realistic target since Mario is the strongest.
May be only when after he finally succeed in defeating Luigi, he'll make Mario a target rival too.
One at a time.
