People don't really care about cliches. What people like most makes most money
cuz most readers (weirdos) project themselves in place of the uke (again, weird as fuck). this is also the reason why most of the bls are written from the pov of the uke. ive even seen some poeple refuse to read a bl if its told from the pov of the seme. (some) people consider the uke akin to the 'female' lead in shoujos, and project themselves in place of them. you'll rarely see shoujos having a love triangle between two female leads & one male lead, and you'll also rarely find a love triangle in yaoi having two ukes and one seme.
the reason whenever it's two uke one of them is evil is because the ukes play the 'female' role in these stories (these tropes are usually written by straight women as well), and if you've ever watched any asian soap opera you KNOW women are all jealous bitches who hate each other. women can't stand to see another women like the guy they also like, so they plot against the other woman to take her out of the competition. /s
and since the ukes or the bottoms are the ones getting fucked (like most women, thus playing the role of the female), they also hate each other, cause problems for the other uke who is their competition.
so, the short answer to why in LTs one bottom is always vindictive? it's a tale as old as time, babe: misogyny.
Why are love triangles always 1 uke and 2 semes?? Why can't it be 2 ukes and 1 seme? Why are both semes portrayed as good people but if its two ukes, then one of them is always evil.
Arent authors tired of writing boring cliches?