Why is the 5G rollout so problematic in the States but not Europe?

What problems are you referring to? As far as I aware US seems to be doing fine, other than have a different mmWave strategy than rest of the world. If you are asking about aviation and specific spectrum, they are tiny if not irrelevant in the grand scheme of things.

Europe has a quite different mobile telco landscape, and avoided certain issues (like aviation impact) from start instead of pushing into dangerous ground.

This means both a more level playing field and a lot more competition, but also less regulatory issues like USA has with 3.8-3.98GHz spectrum.

Because the FCC sold off an infrequently used guard band adjacent to that used by Radar Altimeters, for use by the 5G providers, which was an incredibly bad idea, with predictably awful, and life endangering results.

The root cause is the idea that we should "sell" the airwaves to cell phone providers. It's a tax on us users, and leads to bad outcomes like this one.

5G already uses a wide range of frequencies, they didn't need this particular band.

Like with airplanes?