You can head on over to the official site if you wish to read the full announcement but we have compiled together the most important aspects from it for you guys:
"Showdown was a relatively short-lived experience for most players. Tons of players tried Showdown out when it was first released, but, after only a week, players worldwide spent less than 1% of all in game time on Showdown. Generally, we heard good feedback about the 1v1/2v2 gameplay on Howling Abyss as compared to Magma Chamber. Players also had ideas about continued improvement of the 1v1/2v2 experience such as being able to do specific top, mid or bot lane matchups on SR."
"Ultimately, we concluded that Magma Chamber simply isn’t the right place for us to further develop a great 1v1/2v2 experience and the mode itself isn’t sustainable as a permanent feature. So, after many discussions and a lot of fact checking, we’re halting development of the Magma Chamber map you saw at last year's All-Star Game."
The way Riot has explained this sounds very reasonable, lets hope we get to see some awesome content in replacement in the near future!