Edit: Don't believe any of this, but France was a leader of military fashion, so that much is true.

There is some debate as to WHY certain nations did this. There are two thoughts concerning this: one of emulation of fashion and the other being emulation of competence.

Of the former camp, is Desiree Beauharnais who wrote La Victoire à Travers la Mode: Comment la Mode Française a Gagné des Batailles. In this magnum opus, Beauharnais writes:

As a result of Napoleon's victories in the Napoleonic Wars, France became a leader of military fashion. Before the Napoleonic Wars, military fashion was very basic and bland. The French and Austrian army wore white coats into battle due to cheapness while the Prussians wore dark blue versions of the same style. Military style didn't differ until the French Revolution when the Revolutionaries used blue coats that were cut very differently than the white Royal army.

From this, fashion conquered Europe. Rather than fighting in restricting and embarrassing uniforms, the French were happy to fight in dashing and often sexually attractive uniforms. This is no better seen than in Marshal Joachim Murat, also known as the Dandy King. Spending millions of French francs on personal uniforms, he was best known for having outlandish and extravagant uniforms; famously having a large white plume reminiscent of Alexander the Great.

After the Revolution, many countries were eager to replicate French elan by copying the French cut of uniform. It is best seen in the uniform of Simon Bolivar, revolutionary that set South America aflame. Eager to unify and fight the Spanish authorities. This replication of French cut continues into the 19th century until the upset of Vauban when the Prussians defeated the higher quality French forces. After this, Prussia was the country to copy.

However, there is dispute by a much older source. Written in the 1890s, Sir. John Walton wrote *Copying the Civilized: How the Savage Tries to be Civilized

Since the success of the Ogre, the world has tried to emulate him. The South Americans tried to copy him in hope of creating a unifying South America. However they remain divided due to their inability to join the civilized world in government.

However the Turk and the Jap have taken a look at the French monster and thought that they could replicate his skill if they too looked like him. This narrow minded and simplistic view of the world simply shows that they are not fully aware as to how to win a war.

Unlike the straight forward and clear headed Britishman, the Chinaman and the like do not have a long history of military infighting as the civilized European nor have the light of Christianity to show them the light of gentlemanly actions in war. So they look at those whom were successful and try to emulate them rather than gain the necessary skills.

The latter monograph is highly racist but tires to make a point that is present within European thought, that the non-Europeans are trying to win like a the Europeans. However it's very close minded and racist so it must be taken a grain of salt.

Edited for formating.

Second edit: this was a joke, a highly racist joke about military fashion.