Nuke is one of the all-time classic and enduring maps in Counter-Strike, and has been in use in competitive play pretty much ever since the map was introduced in early versions of the game.
The map is also part of Counter-Strike: Global Offensive, and was part that game's competitive map pool until about a year ago. Valve pulled it in March of 2015 to make room for the new and reworked Train (the CS:GO competitive map pool only ever includes seven maps).
Since then, Valve has reworked Nuke from the version that launched with the game, giving the map a rather gorgeous and splendid-looking visual overhaul along with some tactical tweaks.
Valve has now moved Nuke into the "Active Duty" map group, meaning it will be used in future Valve-sponsored events - and that more or less dictates that it will be used in all serious tournaments.
It's a move the pro players aren't happy with. Players like Na'Vi's Zeus, Fnatic's Olofmeister and JW, and current Major champion FalleN from Luminosity have turned to Twitter to express their disappointment and frustration with the move.
The criticism boils down to two things. One, the new version of Nuke has several technical issues that hurt framerate and performance in certain parts of the map. In a game that's very dependent on pixel-perfect precision and split-second timing, that's a problem - particularly since top level players want framerates in excess of 300 FPS (which is very achievable despite how outrageous it may sound. CS:GO is an old game).
Second, and this is perhaps the bigger issue, is the map that has been dropped to make room for Nuke - Inferno. In short, Inferno is one of the most cherished and popular maps in the game, and one that has hosted some incredible and memorable games.
Given how great Inferno is, fans could hope for a Train-style overhaul of the map (the reworked Train has turned out to be very exciting and popular among players), but given the slow pace at which Valve makes changes to the map pool, Inferno will likely be out for a while.