EA hasn't given an explanation for retiring the games. It did say, "we have had an amazing journey with you so far but sadly, it's time to say goodbye." The decision likely comes down to licensing. EA was once the exclusive Tetris license holder, but that expired a while ago, and it appears EA just doesn't want to renew. Existing users should be able to continue playing the games until they expire in April.
When Tetris turned 35 last year, we called it one of the best puzzle games of all time, partly because of its enduring popularity. EA's decision to retire the app will undoubtedly upset diehard fans. Though, fans can choose from plenty of other mobile Tetris games. Tetris Royale, the mobile title by N3TWORK and The Tetris Company that will include a 100-player battle mode, is in development for iPhone, iPad and Android, and it should be ready sometime this year. If you have a Switch, you might opt for Tetris 99.
In a statement posted in the App Store, EA wrote:
Hello Fans, We have had an amazing journey with you so far but sadly, it is time to say goodbye. As of April 21, 2020, EA's Tetris® app will be retired, and will no longer be available to play. Kindly note that you will still be able to enjoy the game and use any existing in-game items until April 21, 2020. We hope you have gotten many hours of enjoyment out of this game and we appreciate your ongoing support. Thank you!