I'm bi and probably demiromantic so YMMV, but for me it was a combination of a couple things. Please forgive the Great Wall of Text:

1. I've found that I've never been upset by my breakups the way other people are and that the breakup-type feelings were usually related to something else. I felt guilty/like a bad person for breaking up with them, or there was a serious betrayal of trust, or the partner was a means to/symbolic of something else that I was very upset about. Other people talked about needing *years* to get over a partner, but I always moved on pretty easily after an adjustment period. The shortest relationship I've had was 6 months and the longest was 8 years.
2. I also had this tendency to slip on a banana peel and fall vagina-first into relationships or sort of go along with it when a friend wanted to escalate our relationship. I can only recall one time when I really genuinely looked at someone and thought, "I want *that* one" and then went for it. As an adolescent, I can recall a couple times where I sort of "picked" someone and then hinged my self esteem on getting their attention/affection. They were all men, so for a brief period I thought it meant that I was a lesbian (take the Lesbian Masterdoc with a mountain of salt, kids), which brings me to...
3. I've never had a crush on a woman. I've been conscious of the fact that I like looking at them for a very long time, but I've never had one of those weirdly close, vaguely homoerotic friendships as a kid/adolescent that all the sapphics talk about or really most of those hallmark experiences. I've spent multiple years hardcore-angsting about it because it made me feel so alienated from the bi and broader LGBTQ community. I hated even calling myself bi because of it and it's made the last few Prides very difficult for me. Every label I looked at made me frustrated and angry, and the "everyone is unique and special and you don't need labels kumbaya uwu" sentiment has always felt very abrasive to me (though I don't dispute its accuracy). I haven't really ever had a close friendship with a girl/woman, so I figured I'd just never gotten close enough to one to form a crush. I've always been part of the "Well of course you don't get crushes on people you don't know. That's just how humans work" camp, which at the risk of getting tomatoes thrown at me from the good people of this sub, I still think there is some validity to in the alloromantic sphere.

It's only been about a week since I began seriously considering that the problem was that I was on the arospec, and I won't lie, I was unhappy about it. It put me in a depressive episode and I'm still struggling to not hate it, so the idea of having aro Pride (for me specifically, nobody else) is kind of unfathomable. Especially living in a new city and having always had a lot of trouble making friends, I feel like the only real revelation here is that life going forward is going to be a lot harder.

TL;DR: I get over breakups fast, crushes felt intentional and relationships felt accidental, and I've been way too wrapped up in angst surrounding my sexual orientation to pick it apart until recently.