Matt once said that there are few fields because he felt like adding too many would make it feel like filling out a form to add a game to one's collection, and people wouldn't want to use the site if it felt like a chore. Since we have many requests for such specific fields, it would indeed become much like a long form to fill out.
However, while I agree with him, it seems like new fields are requested regularly, I guess it would be worth it to try and find a way to add more possibilities without cluttering the space. I can't promise anything, but I'll add it to a list of things to take a look at.