Because I like the shortbow, and I don't like the crossbow. Much in the same way that I likes swords, but I don't like flails. There's just some weapons I like more than others.
The reason I play D&D, almost the entire reason, is to play the characters that I like. If I wanted to be forced to choose from a set of pre-chosen archetypes, I could just play video games.
If 4E is taking away some choice from me in playing what I want to play, that's a big deal. And again, I don't think I'm being unreasonable here. I can understand the game not supporting an effective, dancing, liquor-hurling fighter. But a shortbow wielding sneaky thief type? It looks like I'm gonna have to make some house rules to play that character at a solid level of effectiveness. Hopefully that's the exception, and not the tip of the iceberg.