Blame the 90's anime writers*
Can you elaborate on that? I always thought it was her decision
If you watch the show and read the manga, Chibiusa's 'crush' on Mamoru was projecting her insecurities. He was a guy who treated her, a 900 year old in a child's body with such affection that she took it the wrong way, once she became less insecure and became a senshi, she developed that effection for Helios. Naoko made it a part of her development as a character.
In the 90's anime Chibiusa's obsession is to the point of Usagi viewing her as a romantic rival and not as a little kid, in the manga she briefly did as she was struggling with her friends getting kidnapped at the time, and by the third arc doesn't view her as a romantic rival. Basically the 90's anime destroys her character arc
