Why does the girl I am not interested in keep texting me, and the girl I am interested in doesn't?

For some reason, I feel bad for the person who is interested in a person who is interested in someone else (like in your case, I feel bad for the girl who you're not interested in, especially if she's interested in you). To explain more, I feel bad for Person A, because they are interested (platonically or romantically) in Person B, who is interested (platonically or romantically) in Person C instead of Person A.
Person A is trying to get to know Person B more, but since Person B is only interested in Person C, it's like there's no point in Person A trying to get Person B's attention. And I find that sad (not out of mockery, but out of empathy).

But if Person A is a bad person (their actions, personality, etc), then I won't feel bad.
If Person A is a good person, but Person B is after Person C, a bad person, then I'll feel even more empathy for Person A.
If Person A is a good person, but Person B isn't, I may feel a slight bit of empathy, but just overall sad; they want someone who will only harm them, wether that's mentally, emotionally, or physically.

But since I am going to use hypothesis, I'm just going to assume Person A (the girl who is interested in you), Person B (you) and Person C (the girl who you are interested in) have decent personalities and are decent human beings; I feel bad for the girl who is interested in you. At age 12, my boyfriend saw a girl a year older than me, more prettier than me. He asked me if it was ok for him to ask that girl out after asking me if I'm ok with a threesome-relationship (poly, is the term. Back then, I didn't know either lol). He asked the girl and she said no. He then asked if it was okay for him to date her instead, so I said it was okay for him to. I said that because I knew he only dated me because I was the only girl in the class at the time.
And also, mostly because as a child, I never asked my crushes out; mainly because I didn't want to seem as a Pick-Me Girl (to better describe it: I didn't want to be hated by the girls in my grade, just in case they liked the same guy as I did. I was a little kid at the time too.