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Cody Rhode's whole story and whole character is that HE. NEEDS. THAT. BELT.
You don't want to give it to him immediately. If you're booking things right, there should be plenty of juice to squeeze (And no, that doesn't mean more Seth Rollins rematches)
Sami Zayn, on the other hand, is in a very unique position where he is currently white hot. You can take advantage of that. Strike it while it's hot, so to speak.
It doesn't mean Sami will be The Guy of the company. It just means taking advantage of what's happening, giving him a nice reign, and then moving on. Sami is 100% capable of that and it's the most interesting decision WWE can do.