Full statement from Nintendo “In the smart-device business, Mario Kart Tour was scheduled to be released this fiscal year, but in order to improve quality of the application and expand the content offerings after launch, the release date has been moved to summer 2019. As we endeavor to develop future planned applications, we will also focus on continued service operations for applications that have already been released so that consumers can enjoy playing them for a long time”

Via Nintendo’s latest financial report (PDF)

The game was previously planned to release by March.