...Because He does not want people to believe in Him just because they saw a “miracle”. He wants them to believe in Him and have faith in Him always.
Plus, there's always the "out" that even if a miracle /did/ occur, people would just forget about it and go on their normal business anyway, especially if the required changes due to the "miracle" upset the status quo too much. Which I think is human nature and would probably happen, even if God appeared in some form on national TV, all surrounded by fire, simultaneously speaking in all languages and it was caught on tape. "Nice effects! Did you get ILM for that?"
However, for the more mundane purposes of the Challenge, the claim has to be testable. If your spirit wants you to win $1 Mil (to give to a charity, to use for your own mission, etc) and to gain some recognition, then your spirit will (hopefully) go along with the test.
If not, then what you're left with is faith, not proof. Personally, I'm one of those folks who does not have a problem with faith, as long as people don't go trumpeting it as fact: "fact" involves science and requires validation. "Faith" (or "Truth") is philosophical in nature and is fine as long as it does not stray into the arena of "fact" without some form of testability.
So if this ends up being a test of your faith, or if the spirit requires you to believe in He/She/It despite a lack of objective testing, that's fine...just please don't go telling people that you have access to "fact" through this spirit, or that the existance of the spirit itself is a "fact".