Why are the losing heroines the best girls everytime ?
EVERY
GODDAMN
TIME
Main heroine generally can't make any progression or she loses the author's central appeal for her, so she must stay bland and sort of disconnected, meanwhile he adds a second or further love interest who is permitted to be developed properly and gain actual real intimacy and clear romantic chemistry. So after this whole process has played out, the author has to claw it back to his original goal and nuke her chances with some plot device or contrived situation, because how dare people like anyone other than the authors designated waifu... errr sorry designated winner. So that's at least a part of why best girl always loses and romcom authors are sadistic fuckers cynically manipulating their readers to invoke drama and sales.
Yamamoto also seems to use this tomboyish Rin character type as a sort of fall girl, take his series "Fudatsuki no kyoudai" about a boy and his vampire sister (spoilers necessarily ahead so if you've not read the manga decide for yourself), the character Hikari becomes friends with his sister but during the course of the series realises that the guy she was in love with is Fudatsuki-kun, meanwhile he's in love with the seemingly braindamaged class president character always worried about his reputation and how she will see him, in the end Hikari never actually confesses and gives up after a sort of confused confrontation, another character in the series is called "Rin", the older female cousin of the two siblings who has a lot of bad luck with men. In another of his series "Kunoichi Tsubaki no Mune no Uchi" you have the character "Rindou" who is the butt of a few jokes and shares character and design traits with both Hikari and Rin.
The only series I can think where the author actually has the tenacity to give side girls a real chance was "bakutachi wa benkyou-sama" by tsutstui tachi. because Kirisu Mafuyu became an absolute firm fan favourite (her popular vote scores completely dominated the character polls by a ridiculous margin) so the author structured the series to at least make it possible for her to have an ending by creating an omnibus end of multiple routes. According to the author Tsutsui's interviews her popularity really came out of nowhere and surprised him so much that he felt he had to instead of being the side love interest. Some people try to suggest Botan from "Ookami Shounen wa Kyou mo Uso o Kasaneru" but she was likely clearly established from the start as the main love interest after the set up.
Long post but I hope you don't mind.