i used to be like "why do people always stop being friends w their exes, if i broke up w someone i would simply stay friends" but i get it now... when you're dating someone it's so easy to let issues slide rather than turn it into a whole issue, until you finally do break up and then you let yourself get upset about all the old problems and at that point why the fuck would you bother putting in the effort to repair them?
really it's less that breaking up ruined the friendship, and more that you already had stopped working as friends, but instead of slowly and naturally drifting apart you stuck together for too long until there was no amicable path left.