You've to appeal to their spreadsheets. Generally, lower costs.

If Oracle is well-established, it's going to be tough. It isn't just the cost of Oracle and doing business with them, it's the cost of all the work associated with making the transition. Code-rewrites, retraining ops, and so on.

If there's an opportunity for a greenfield, small project that would benefit from having a distributed backend then try that. Be prepared to show that it's not just low-cost, but it is easier to implement.