“And that standard Mexican Spanish seems to be the clearest and most straightforward to understand.”
I’m far from an expert on it but seems that way to me, too. Maybe just most familiar?
I was thinking that initially but as I listened more and looked up Mexican films, speeches by Mexican politicians, etc...It seems to be objective observation. I grew up around a lot of Cubans for example and they speak very differently.
Also a lot of immigrants in the US are from countries in Central America, not Mexico.