Michael Acton Smith, the founder of Moshi Monsters, has decided to close down the site because of reasons out of his control. adobe was going to shut down flash (what Moshi Monsters runs on) anyway in 2020. Michael Acton smith decided to shut down his beautiful creation before then so it doesn't cause glitches and problems. And to make sure the Monsters get a nice, quick, painless death, as opposed to the alternative; a very slow and painful one. if it wasn't shut down before flash was. Fortunately, this isn't the complete end to Moshi, as they are still going to continue the two mobile games that they created.
The six monsters aren't going to feel too upset that they are going to die though, as they get to join their "friends" Club Penguin. although i prefer Moshi Monsters over club Penguin because there are more things you can do without needing a membership.