Fact is that WWE still doesn't know what they have in Reigns yet. What if his title run bombs? What if, by this time next week, we're all complaining about some horrible promo he did. What if he gets booed out of his next big match? What if ratings continue to fall?
If Cena is to turn heel, it can't be until the new "guy" is established. That hasn't happened yet.
And in case Romans push does fail (which is most certainly a possibility, as it has fallen pretty flat up to this point), then WWE is going to need Cena to fall back on.
From a business standpoint, it really can't happen until then (as much as I want to see it).