When I think about the US, the majority (not all of course) of our biggest cities are coastal or are big ports:
Chicago (on great lakes)
Washington DC (Chesapeake bay)
For Mexico (of which I am not an expert), it looks like their biggest coastal city is Tijuana, which is roughly the population of Virginia Beach. I am not even sure what the next one is, Veracruz? This is for a country with 128M people! All their bigger cities are internal.
I am sure there is history here. Lack of immigration from the old world? But it seems like first settlements in the New World should have created more big cities on the coasts?