People prefer Modern Amy to Boom Amy becaue Modern Amy is a bit naive, helpless, and most importantly predictable.
The thing about girls with personalities like Amy is that once they grow up they are always unpredictable, and some people can't handle a girl who is unpredictable, nor do they want to accept that a girl is not what they thought she was.
But.. if you can handle a girl like her and her unpredictability, and you have her on your side, she can be a powerful asset, because the world is unpredictable, as is combat. And with time, she will also come to show the best of her true feminine side only to you