derefr has a great explanation of traditional vs proper vs authentic. But on Why do only some cuisines have such constraints?, that's probably just down to culture.
Prescribing how a dish is made expresses a deep cultural identity through food, even if that food has evolved greatly over time. You might be able to change a dish and even improve its flavor, but it would no longer be that culture's food.
Now, why do only some cuisines seem to be sticklers more than others? Probably because some people are more sensitive than others about their food. They probably went to get a certain kind of meal, and it wasn't what they were expecting, and they were pissed off. And I get that. If you have an intention to eat this culture's food, you don't want some weird western "interpretation" of it, or some jackass from NY who thinks putting cream cheese on it makes it "regional". You want to be connected to where that dish first came from, and all the breadth of history that distilled into creating its specific flavors and textures. You want that dish.
It doesn't help that when dishes are transported out of their country of origin they get transmuted into some kind of local monstrosity. Tex-mex? Spaghetti and meatballs? "Chinese takeout"? California rolls? It's clearly not the real mccoy, but people still call it "mexican", "italian", "chinese", or "sushi", when it barely resembles those cuisines at all. And don't get me started on trying to find a real burger or bbq anywhere outside this country, to say nothing of those rarer American foods, like drip coffee.
And to be honest, there's probably a certain amount of cultural elitism involved in "cuisine" (the French word for "we're better than you"). I've always found the French and Japanese alike in that they both think they're superior to everyone else on earth. That's probably going to result in a lot of uppity people wanting their dish the "superior" way. But for me, it's about wanting to understand and respect the deep cultural traditions that produce singularly unique and beautiful expressions of food.
Yes, coffee is made in many different ways around the world. The French, for example, only drink drip coffee at home. The Viennese famously have it strong with whipped cream. Much of the Mediterranean uses an Ottoman-derived concentrated boil method. Trying to find a drip coffee in these places, and many other countries, can be quite difficult.
