Why she's on the list:
Madonna might have finally aged out of pop music relevance, but she's reportedly sold almost 300 million records in her career, and her best hits from the 1980s and 1990s are modern classics that surely would have made a good amount of money if licensed to Rock Band during the music game's peak. (And yes, again, Madonna makes way more sense for Rock Band than Guitar Hero.)
Why she may not be in the game:
In a way Madonna's already in Rock Band, as several Lady Gaga songs have been released as DLC. Madonna's nonexistent Rock Band presence might be due to the poor timing of the keyboard peripheral.
It'd be hard to nail the Madonna experience without keys, but by the time Rock Band's keyboard came out, the genre had already bottomed out. Unless the still raging stream of Rock Band DLC eventually includes a Madonna pack, we'll have to make due by singing the lyrics to "Express Yourself" over the Rock Band version of Gaga's "Born This Way".