XFLAG and Mixi announced the service of the Monster Strike but only for the English version. In Japan the game is one of the No.1 smartphone game, so the Japanese version users don’t have to worry about anything but not the English version.
In North America / Europe the game wasn’t that successful like in Asia. The English version Monster Strike released in 2014 and you can play with it until August 1, 2017. After that you can’t login to it. The last update going to release on July 2, 2017 and you have to note that you can’t ask any refund.
According to Mixi and XFLAG and reason is simple: it wasn’t as successful as they planned it.