If I had to guess, it's because of what what nicho noted about Exchange: until it knows you're using iCloud for a list, it limits the functionality to what's compatible with syncing with other services like Exchange, so that if you were to enable syncing with such a service in the future your existing reminders won't lose information/functionality/their structure.
Apple has obviously decided that instead of letting you use all the new functionality and then having to provide a way to handle what will happen if you then later try to sync it with Exchange (e.g. you add a photo to a reminder, then turn on syncing with Exchange and it loses the photo), they'll prevent you doing it up front.
This makes sense, as the vast majority of people have Reminders syncing with iCloud (it's the default behaviour), so this is the trade off for the minority case. It's not like you're losing any existing functionality here, this is new functionality they're introducing. If it really is a "game changer" for you, toggle the Reminders switch on iCloud and enjoy all the new stuff.