Cash Cow Franchise: The first game holds the virtue of being among the original 17 "Sega All-Stars" games for the ill-fated Sega Dreamcast, and the second appeared on the PS2's "Greatest Hits" list for a time.
Doing It for the Art: Word of God said that Michael Jackson refused to be paid for his involvement in the game, content with just being a part of the game instead.
Keep Circulating the Tapes: Physical copies of the first two games have gone rare. Even with the massive increase of interest after Michael Jackson's death, physical copies (especially the Dreamcast ports) are hard to come by.
Screwed by the Lawyers: in a similar vein to his guest appearance on The Simpsons, contractual issues prevented Midway from properly crediting Michael Jackson as a voice actor.
Also, thanks to potential licensing issues between MJ, Shaquille O'Neal & the Clintons, WB Games (the current owner of nearly all Midway properties, including Mortal Kombat), an Updated Re-release of Round 2 likely isn't going to happen for a long time.