1) Their personalities when around each other is already seen in many other fictional pairings, they're both tsundere for each other, or atleast a subset of tsundere. At first we can only see the tsun, but me explaining the dere would spoil the movies.
2) They have rather similar designs, messy hair with a constant angry facial expression.
3) Every piece of official art outside of the movie has them always close together. This could be promotional art, or special extra posters that comes with pre-ordering the BD, and even in the gacha game pin-ups they seem to always have a magnetism towards each other.
4) This is the French school, they are larping as Frenchies. Rose of Versailles is, or was, an extremely popular anime/manga that took place in France, and from this anime there is a popular ship, Andre x Oscar. Andou and Oshida are based on these characters.
5) The artists that draw this couple seem to be very dedicated to them, some only drawing these two and by chance or design this ship seem to have attracted some rather talented ones. Through my excavation of Pixiv and Twitter, I also notice a significant number of exclusively het artists who makes one or two yuri works in their entire catalog, and it's always these two. You can probably safely say many people like the artists rendition of the pairing.
6) Further expanding on the point above, from what I have seen, it appears that the artists that draws GuP comic in mass published magazines have accepted and incorporated the yuri element of these characters into their work, even the latest Motto Love Love Sakusen Desu, an official comedy spin-off of the series, has them in a rather romantic pose on the cover.
7) Blonde x brunette, rich x poor, peasant x royalty, rebel x goody-two-shoes.
8) "Just as goons to another character" is a rather reductive way of characterizing them, considering the series they are in. GuP has alot of characters with little screentime that mainly function as obstacles against the main protagonist. By your logic, the most popular characters that comes out of the series are also just goons to another character. Maybe a better exercise is to see how the characters work in the their own respective schools. They are pretending to be French, and France has a history of internal conflicts and civil riots, I'm sure there's something going on there right now even. These two characters represent that. If, lets say, they ARE indeed just goons, and the don't have their own characters, then what's the point of having a French school?
9) I just thought of this point in the middle of writing the previous one, so it might not have gone through some required consideration, but to add on to the "most characters have little screentime" detail, I do believe that the less specific they present characters, the more empty spaces artists, fanfiction writers and daydreamers can fill. This applies to all of the minor, supporting characters, but why this couple in particular gets the most amount of dedicated imagination is probably because how much they interact on screen, through verbal and physical means. We know that they seem to be the leaders of opposing groups of people, but we only see one riot in the movie. Obviously there has been and will be more, so the fans already have a colored canvas that they can slowly add details to.
10) Bruh this shit good fam, dont question lmao.