I've been watching lots of anime series and in lots of them the guy realizes that he loves the girl when he sees her in a kimono specially for the festivals. Just this last week it was both in ReLIFE and Twin Star Exorcists. Just what is with that?
Just a small correction: guys do not simply fall for the girl when he sees her in kimono/yukata; his love was built up and the kimono/yukata just sorta delivers the final blow.
The kimono is a traditional Japanese clothing, it shows elegance and beauty to the people wearing it. It is a formal garment that can be worn to weddings, etc.
For teenagers, they don't get the chance to wear something fancy often, and the kimono is something fancy which the teens can wear once or twice a year. Because it's rare to see people wearing kimonos, it excites people psychologically. It's sorta like finding a shiny Pokemon, all it is is a different color with a low chance of encountering and people treat it like gold. It's the same with this, because of its rarity, it enhances the beauty of the girls wearing it.
Like I already said, this will only work if the guy has some feelings towards the girl. If a random guy saw a random girl wearing kimono, he might feel attracted but wouldn't fall for the girl. And the trope can work with other fancy garment as well, such as a wedding dress (they go to a park where the girl tries on a wedding dress) in some anime, or a qipao (Chinese dress) depending on the anime. Most of the time it's a kimono because it's easy to relate to and it's in Japan.
It's a similar trope to one you see in Western media, where the normally un-feminine (or dorky, or nerdy, or otherwise not stereotypically attractive) female character dresses up for some kind of fancy party - if the characters are school age, then it's frequently a prom. So they ditch the t-shirt, jeans and sneakers, don a dress and heels, swap the glasses for contacts, maybe put on makeup and get their hair done, and then become the belle of the ball and make the male protagonist swoon.
A kimono (or yukata, since a lot of these scenes take place at festivals in the warmer half of the year) is a very traditional dress, and it's associated with elegance given its fabric (often silk) and intricate patterns. In addition, when wearing one, women will often put their hair up, maybe put on a little makeup, and wear geta which are elevated sandals, which have an effect on the wearer's height and posture similar to wearing high heels.