They mentioned it in an interview with Stadiacast. The answer is pretty bland, but somewhat points to them not trusting LTE as a source for the time being. "There's a lot of bad LTE out there" I think he said.
If that just means that they'll wait until after the review-period or if they are actually waiting for 5G is hard to say. In Sweden we don't have data caps on Wi-Fi. we do however have them on mobile, and even the "unlimited" carrier deals have a limit per day. Usually around 20GB, and then you have to fill it up though text-messaging every 5GB.
I think one factor preventing Google from allowing LTE to work (without some tricks, anyway) is their concern for players exceeding their data caps due to not understanding how much data they are using. Like @Bakken said, while some players may have access to "unlimited" LTE, usually after some amount it is throttled to a point where it would be unusable for Stadia.
The far worse case is players incurring massive bills from overages. On a home Internet connection this is bad enough, but at mobile data rates it would be absurd. While not Google's fault, it is a bad look when their service is leading to news articles with headlines mentioning how someone's kid using Stadia cost the parents hundreds or thousands of dollars in overages.
I hope this changes soon. A lot of people have unlimited data on their cell plan, and GeForce now let's you use LTE. I was just playing Division 2 on my sprint LTE in the airport and it ran beautifully. Better than it does at home. There's no reason Stadia should keep preventing LTE usage if the connection is up to par.
A failed attempt to connect is much better than not having the option at all.