I don't get it. Some people say that if a guy/girl feels like s/he is not good enough for the partner no matter what, then they wouldn't date that person. Usually people like this are the ones that were teased at school or just don't feel comfortable with the way they look. Hence they don't feel comfortable being in attention, why is not wanting attention that unattractive? So you prefer an ugly person who always want to be in the center of attention?
People who don't want attention on them don't necessarily have low self-esteem. That's not what makes a person unattractive. When you compliment someone and they don't believe you no matter how many times you tell them it's true; when they don't see their own self-worth; when they don't even understand why you like them, they become less attractive. It's frustrating and difficult to be with someone like that.
It's not that low self-esteem is unattractive (at least to me), it's just that it makes someone a lot harder to date because they don't see themselves the way you see them. If you compliment them they just brush it off and get embarrassed. If you like someone who has low self-esteem, they don't believe that you like them and it makes it much harder to progress.
There is a definite difference though between being a low-key, quiet individual and one with poor self-esteem. The quiet individual just has a more soft-spoken personality and doesn't want to be the center of attention, that's fine.
The person with low self-esteem will often push aside sincere compliments that are given to them and treat themselves like dirt, which in turn will make other people see them negatively by default. It's a self-fulfilling prophecy.