I would expect that people who make money on Nostr will be interested in a good infrastructure. So if McDonalds sells burgers on Nostr they maybe want to make sure that there are some working relays. 
There could also be some enthusiasts like the people on the fediverse right now.
Third option I see are payed premium relays which offer special services.
 

 I am also thinking that trading communities would be willing to run their own releays or someone in that community would like to sponsor one.