I'm sorry, i should have been more clear. In the US we have two competing cellular technologies, out of the five biggest carriers in the country, three of them use GSM/UMTS+LTE/HSPA, and the other two use CDMA+LTE. When i refer to a GSM-only phone, i mean that these phones are incompatible with three of our biggest mobile carriers here. The issue here is that carriers such as Verizon not only use CDMA as their fallback/low-bandwidth radio, but they also use a different LTE band than all GSM/LTE phones do. For example, even though my Lumia 950 XL supports LTE, it doesn't support LTE band 13, which is what Verizon uses as their main LTE band. And this is typical of all "world phones". Confusing, right? Add to this that Verizon requires every phone that supports their frequency bands to go through an expensive certification process. In Europe everything is pretty much all GSM-based but here in the US is a different story. We have all these antiquated CDMA networks that are going to take years, if not a decade or more, to completely replace with more modern, widely compatible technologies such as VoLTE. This has little if anything to do with SIM-lock, as all Verizon-certified phones are unlocked from the factory these days, and you could actually use them on a GSM carrier, but unless they are fully GSM capable they will operate with reduced functionality on those networks.
The main point of all of this is you can't just slap an LTE modem and an antenna into a device and have it magically work on all LTE networks. Unless, of course, you want to alienate your US market. A lot of people here depend on carriers like Verizon because they simply offer the best network coverage here. That is why, and hopefully this statement makes more sense now, "It is hard to make a phone/device that works on every carrier". Because it literally requires large amounts of money and jumping through hoops to do so.