Million dollars is a big money for a person, but small for a collaboration. The point of giving big money is to motivate people to work hard on big problems. It's not such a big motivation if you think: If I work hard, perhaps my big collaboration will one day get million dollars.The prize can be given to collaborations or institutes, it just has never been done. I guess they just don't want to do that for whatever reason. Picking (at most) three experimentalists from the discovery would have made no sense, the discovery was the work of hundreds to thousands.