Why do trucks hog right lane

Its so irritating when the slow moving trucks and buses hog the rightmost lane on the NICE road.They slow down the entire traffic and force faster moving vehicles to weave in and out of the fast and slow lane endangering everyone.Why do they use the rightmost lane I can't see any particular advantage other than just blocking traffic.NICE authority don't even penalise them.Whereas in Kerala I have see warnings of fines for using right lane and the truckers strictly keep to the slow lane.