The US is using a 5G frequency band that is much closer to the radar altimeter band so there is a greater risk of interference.
From here:
There is no single part of the electromagnetic spectrum that 5G occupies. Some countries are using 600 megahertz to 900 megahertz, which isn’t dissimilar to 4G. Some are placing it between 2.3 gigahertz and 4.7 gigahertz, which boosts data speed somewhat. And others are using 24 gigahertz to 47 gigahertz, which requires more towers but offers even higher data speeds. In many cases a network will use a mix of these. In the US, the frequencies allocated for 5G are closer to those used by aircraft than those allocated by the EU.
Radio altimeters operate in the 4.2 gigahertz to 4.4 gigahertz band, and the US has set aside a portion of the spectrum right up to the lower band of that for 5G. In Europe, the comparable band ends at 4 gigahertz.