Why they're on the list:
Sweden's ABBA had their own SingStar karaoke game in 2008, and starred in a Just Dance spin-off that came out last month. You still can't play along to their music in a video game, though.
In a way, ABBA is the most surprising entry on this list. They remain massively popular throughout the globe over thirty years after their heyday, with worldwide record sales supposedly approaching 300 million.
Unlike much of the pop music of the last twenty years, ABBA's songs weren't made with samples and sequencers but traditional instruments like guitar, bass, drums and keyboards. They should've been a no-brainer for the keyboard-supporting Rock Band 3, especially when you consider their vocal harmonies.
Why they may not be in the games:
Their video game rights are probably tied exclusively to the ABBA: You Can Dance game, making an eventual Rock Band DLC package unlikely.