Zelle charges the banks 50 to 75 cents per transaction. It's free for the end user, though that 50 to 75 cents is passed on to the end user anyhow. (probably by offering slightly lower interest rates, or whatever.)
Additionally, Zelle is getting in on data brokerage; like Visa.
That said, it should be noted that that fee isn't so much to make money as it was to maintain the service. Zelle's purpose was to break Venmo and Cash App. (which is why it's integrated into your banking apps.)