There's actually a easy explanation for that. I had a theory I had awhile back that MJ was just using her supposed acting job in L.A. as a cover for her activities as Jackpot, "until the whole Sara Erhet business was thrown in. Remember the "Peter Parker: Paparazzi" arc where it was revealed that she was supposedly coming and going to New York in secret without getting in touch with anyone? MJ being thought to be "in California" provides her with an "alibi" when she's Jackpot.
Course, since it's very likely now that Jackpot is indeed MJ, I'm sure some of the reaction is going to be either "I knew it!" or "If Marvel planned on having MJ be Jackpot all this time, why the hell did they string out the mystery of her identity when they could have told us from the get-go?" After all, I think the bigger question would be is how MJ acquired super-powers moreso than who Jackpot's true identity was.
