Zelle can be convenient for trusted reimbursements, small group payments, or local vendor transfers. That does not make it a registration system. It has no ticketing workflow, no attendee database, no checkout page, no seating logic, and no real event reporting.
For public events, the risk profile is the problem. Bank limits vary, international and card payments are out, and there is no purchase protection for goods or services. Use Zelle only when both parties trust each other and the payment is simple enough to reconcile manually.