Because long term all relays will need to have some sort of business model to pay the rent. Actually, paid relays would decentralize more the protocol rather than what free relays do. if there's something free, majority goes ther; if everything comes at a cost, people are going to decide given some criterias they value more. Some will value cheap-ness, thus will use the cheapest relays. Some will value long term storage of notes, thus will pay for relays with longer storage. Etc.