tremere110: I'm guessing he meant "we
couldn't warm users ahead of time".
As to why - perhaps the publishers right to distribute expired and the owners of the IP decided not to sell on GoG and sent an email basically saying take down our game please. GoG complied immediately of course.
For the record - the original owners (WizarBox) of Gray Matter declared bankruptcy in 2012. In 2013 the remnants of the company was transferred to Interaction-games. The publishing agreement was likely still in force so Viva Media (the publisher) could sell the game on GoG in 2014. The publishing agreement probably expired so here we are.
Why is it still on Steam? Maybe Interaction-games wants to keep selling on Steam and transferred rights without interruption. Or maybe Steam hasn't taken down the game yet - it's often slower that GoG in that respect.
Can't say, but Steam does have a history of being the last to delist.