Nintendo wants to improve quality before launch
It was a year ago to this very day that Nintendo first revealed Mario Kart Tour, which brings the kart racing series to mobile devices. The original plan was to release the app by March of 2019, but Nintendo just announced a delay that pushes it back to summer.
The announcement comes from Japan, and there's currently no followup from other regions. According to Nintendo, the delay was put in place so they can improve on the game's quality and expand upon the post-release content plans.