They could, but its expencive to give out paid Steam keys, unless they made a deal with Steam somehow, so they not wasteing extra money for the keys, that they would need to give out to every person who bought it on Itch.io before its release on Steam.
Logicaly they should not give out keys to people who bought it on Itch.io since that would not make sense. But people who supported the game on like Subscribestar, would more then likely get a key, since they technicaly helped with the games funding to make it possible to reach a full release for like Steam.
Yes, you bought it on Itch.io, but those on Subscribestar, they paid it monthly so it would be possible.
Big difference.