People won't pay the ransom if they lose their data anyways. So since they no longer accept payments from [X] ransomware they released the key. This way when they infect people with [Y] ransomware they are more likely to pay.

It's this weird sort of racket where they hold to their word (they'll give you a key to unlock your data!) but they're the ones locking away your data to begin with... but people are more likely to pay if they hold to their word.