After enabling iOS COVID exposure notifications, why was I not prompted to enable Bluetooth?
I would go back to basics. How are you disabling Bluetooth? The radio is active for system services in almost all cases and I would expect enabling COVID 19 exposure notifications implies that each device hardware listens and broadcasts on a separate hardware channel than any other Bluetooth emanation or pairing.

https://www.eff.org/deeplinks/2017/10/ios-11s-misleading-ish-setting-bluetooth-and-wi-fi-bad-user-security
https://covid19-static.cdn-apple.com/applications/covid19/current/static/contact-tracing/pdf/ExposureNotification-BluetoothSpecificationv1.2.pdf

Both Google and Apple have the same design document up and someone with a Bluetooth sniffer could validate this as well.