No, MyBitcoin has been down for a while, so no one can pay for bubbles (but maybe they have fixed it now :s)
That is part of it, but I've noticed it dropping quite rapidly. It's a lack of interest by people mostly. If we could find a good way to advertise the site and get more people playing, it will create a larger jackpot and a slightly faster game.
Post your referral links on any bitcoin site you know of!
