alt Hacker NewsIt was also much easier to import cheap unlocked phones from abroad when the whole world used at most 4 frequency bands. Now there's several dozen and I don't think even the most expensive flagship phones support all of them.
To paraphrase a common phrase I've seen around here: The "G" in "LTE" stands for "global".
There are tools to check the compatibility of the phone with the carrier, some sellers like Tradingshenzen integrate that into their shops. A quick check shows that indeed the US carriers have many incompatible bands with the phones. On the other hand, there is only one or two bands that you have to check when importing to Europe.