Because you're probably not her type.
You sound like a nice guy but a pushover. And that doesn't really attract girls.
Also if you two are close maybe she is afraid your relationship will ruin your friendship down the line.
You said it youself "Best female friend". She sees you as a best friend, and when that happens its hard for a girl to see thee best guy friend as more than that. i tried it and it only lasted a week, i could not even kiss him and did not want to ruin what we had as friends.