The primary reason this doesn't work, and something even many spanish speakers in the US don't get, is that there are dialects of spanish where "Latinx" is literally unpronounceable. There are places where "X" is pronounced like "Ch" or "h" and not like "ks" like white people in america pronounce it, for example. When you brings this up, the response is to "just pronounce it like latin-equis." X isn't always pronounced "equis" in spanish speaking countries, that's a very anglo pronounciation. Typically, the more southern or native american a spanish speaking culture is, the less likely "X" will be pronounced the way white people do.
And they don't seem to get how that is cultural imperialism. How insulting is it to have people, not from your home town, telling you to change the pronunciation of the term for yourself, and in order to make it work, for that one word, pronounce the phonemes like an entirely different culture pronounces them. It's double insulting, in a way that deeply reveals some bubbles people live in. Like, people flat out don't get it.
A comparison I made, say for some reason there was a push to rename "Seattle" to "Xiattle." Just pronounce "Xi" like they do in some asian countries, and not as it's often pronounced in english, and it makes sense.