Okay, it's for polish reasons, which in the wake of some high-profile releases this year is an entirely understandable reason to delay the release. From CD Projekt Red's letter to the community:
"The sheer size and complexity of The Witcher, key features of the title, have had a decisive impact on production. Now, nearing the end of our work, we see many details that need to be corrected. When we release the most important game in our studio's history, we must be absolutely sure that we did everything we could to limit any bugs to a level that will allow you to enjoy the game thoroughly.
"With this in mind, we took another look at current workloads and what they mean for the team. Even though everyone is working at full speed, we concluded that we need another 12 weeks, so we are shifting the release date of The Witcher 3: Wild Hunt to May 19th, 2015.
"We owe you an apology. We set the release date too hastily. It's a hard lesson, one to take to heart for the future. We know what we want to do to make Wild Hunt one of the best RPGs you will ever play. And we continue to work hard to achieve just that. So, we apologize and ask for your trust."
And hey, it's not all bad. With The Witcher 3 delayed you'll have more time to play the stupefying number of February/March releases this year-- Evolve, Pillars of Eternity, Batman: Arkham Knight, et cetera.