Lots of reasons, circumstances change, relationships end or start to go badly, their other option got into a relationship, you happened to come into their mind,
my advice is don't hold grudges and don't lead with an ego. Be open. Not everyone is feeling you exactly when you are feeling them. Or the exact same way. Or they are feeling you, but someone is closer to them, or they have more history. Whatever. There are a dozen variables.
I know not everyone is the same, but I can switch attractions rapidly, all it takes is a few minutes, so I never let someone I like/I want pass by me, because of my ego.
also, give the same kind of grace you would want given. my approach to these things is a lot softer than a lot of advice men will give, but to me, so much of that is just based in like belligerence lol they want to outraged and angry, more than they want the person.