Well that one is a bit of a surprise its usually the US Presidents first official visit switches between Mexico and Canada usually.
Perhaps it has something to do with the fact that the presidents of both Mexico (Enrique Peña Nieto) and Canada (Justin Trudeau) hate Trump. Also, Saudi Arabia is one of the most powerful allies of the United States. They deserves better recognition from the POTUS.
Well one was pissed because of a Expletive Wall the other due to the perennial soft wood dispute since it's not hard enough to satisfy both parties
Your right though it just seems like it's a forced habit in modern US state visits. I think though that it is the first Middle Eastern the list of Presidential first visits wiki says islands in the pacific, other European countries, Asia, Latin America but not the Middle East so Trump gets a first.
https://en.wikipedia.org/wiki/List_of_international_trips_made_by_the_President_of_the_United_States