Midway Games closed up shop last year as the company went completely bankrupt. Pieces of its franchises were sold off to various publishers, but a lot of its back catalog remained in jeopardy.
Whatever licenses Microsoft had in play for several Midway XBLA titles seems to have expired, and with no one stepping up to claim ownership, the games are no longer available for purchase.
Among those titles are Joust, Smash T.V. and Gauntlet. All three of those titles are excellent cooperative titles, most offering hi-res graphics on top of the classic game and online play. Other titles de-listed include Ultimate Mortal Kombat 3, Root Beer Tapper, Defender and Paperboy.
All of the games are still available to re-download if you already purchased them. As a side note, a lot of these games still have codes available to purchase on sites like Amazon or via 3rd parties. These codes WILL NOT work. So please don't throw your money away.
Hopefully in time the licenses will get straightened out and we can all once again earn BIG MONEY and BIG PRIZES.