Why they're on the list:
Unlike some of the other artists on this list, Pink Floyd's absence from both Guitar Hero and Rock Band is surely self-imposed. Say what you will about their particular form of overblown art rock, but Floyd remains one of the most beloved and influential rock bands of all time, with certified record sales surpassing 100 million (almost half of which are for Dark Side of the Moon alone).
Why they may not be in the games:
There's no way the makers of Guitar Hero and Rock Band didn't repeatedly back up a tour-bus or two of money for the rights to a Floyd song. This infamously fractious band can't even agree to make an obscene amount of money on a true reunion tour, though.
They also seem like a perfect cross-section of two distinct strands of close-minded anti-videogame absolutism, namely stodgy Baby Boomer narcissism and serious artiste pretension. Perhaps they would've been in line for their own stand-alone game if every single-artist music game hadn't seriously underperformed.