Dark Souls: Remastered for the PlayStation 4, Xbox One, and PC will still be released on May 25. The Nintendo Switch version was supposed to be out on that date as well but it looks like that date has been pushed back to "later this Summer."
The new release timing gives the development team the time needed to make sure Nintendo Switch players can fully appreciate the true DARK SOULS experience anytime and anywhere, thanks to the unique portable nature of the Nintendo Switch console.

Just chalk it up to the game needing more time for some polish before being released.