I guess it's partly that first impressions stick. I was not an Edmund fan until quite recently, when I read elecktrum's writings on FanFiction.Net (especially Black Dwarfs, Blue River), and I have been reading and loving Narnia for over 35 years. Somehow I just never really THOUGHT about his character after my first impressions were formed as a child.
You've probably already discovered elecktrum but if not, here is a link to her page. She has adult concepts, but I personally consider her work suitable for teenagers up, and she is clearly a big Edmund fan. http://www.fanfiction.net/u/450594/elecktrum