Disclaimer: The reasonings stated in this node are by no means comprehensive. Fashion is almost like Brownian motion.
I'm sure you've all seen those paintings of people (especially the French) from back hundreds of years ago. The ones who wore big puffy pink pants, kind of like what rappers in the late eighties wore (except not nylon). These pants were pretty normal for elites to wear back during the eighteenth century and before.
However, we all know that these pants aren't worn anymore. Instead, everyone in Western culture pretty much wears the same type of pants. Either the denim jeans, or fancy version of them (black cotton, etc.). If you're feeling crazy, you might even wear some khakis. Why did this happen, though? Why did people stop wearing those super cool pink puffy pants?
Louis XVI. Yup, we can thank good ol' no-head, the king who ruled during The French Revolution. Well, maybe not directly, but he was a major cause.
You see, the winners of the revolution were the bourgeoisie. The people who run businesses and have considerable amounts of money, but not as much as the elite. These bourgeoisie (along with the less affluent members of society) had no need for super cool pink puffy pants, so just wore more practical leg warmers.
Once the bourgeoisie ruled France, the pants turned into a "do as the Romans do" concept. The previously elite could be heckled, if not killed for almost anything that they did to appear elite, so strived to appear "normal." Part of this normalcy was wearing plain everyday pants.
So, if you think your family used to be a wealthy French family (or other European family), just take a moment to say, "Thanks Louis, now I can wear jeans!"