NEXON Corporation is the developer for Counter-Strike Online with oversight from the original license holder Valve Corporation. The reason why NEXON came to be the developer is due to the fact that Counter-Strike Online is targeted towards Asia's gaming market and NEXON is quite popular around Asia as a game publisher.
Overview
NEXON Corporation (Korean: 넥슨) is a South Korean video game developer and publisher of online games and MMORPGs. Its main headquarters is located in Seoul, South Korea (although it also has four other corporations running; they are located in Korea, Japan, United States and Europe. It is the company that creates Counter-Strike Online with the help of Valve Corporation. Its current and most successful game ever published is 'MapleStory'. Nexon currently has eleven subsidiaries: Nexon Networks, Nexon Mobile, NeoPle, Nextoric, Nexonova, GameHi, NDoors, CoPersons, Nclipse, ExcGames, and Xeogen. NEXON also decides when and what contents to add to Counter-Strike Online (with supervision from Valve).
On April 5, 2012, Nexon and Valve announced a partnership to develop Counter-Strike Online 2, which is to be based on an enhanced Source engine and will offer enhanced graphics, powerful impact physics, and more new features. It has not been confirmed if it will have any connection to Counter-Strike: Global Offensive.
Although Counter-Strike Online 2 is going to be released, it does not mean that NEXON would stop adding new content to Counter-Strike Online. (NEXON will still continue to add more content to Counter-Strike Online).
Also, Counter-Strike Online 2 would NOT be replacing Counter-Strike Online, although Counter-Strike Online 2 is the sequel to Counter-Strike Online, they would still be two separate and different games.