Any idea how much it costs to design/integrate/flighttest/support a second engine type? And comparatively how many/few extra orders (hence profit) this will/might bring? The trend towards sole supplier deals speaks for itself on the cost benefits balance.
Also airlines these days can buy engine maintenance from third parties or can even lease engine usage by the hour and not having to worry too much about maintenance. Some of them even have joint stock of spare parts and they get delivered with a simple phone call. So i think engine commonality is probably not the biggest issue when they buy planes these days.