The game may be linked to a Nintendo Account that is not linked with your save data.
Use a Nintendo Account that is linked with your save data to transfer your in-game progress.
Save data would start from scratch due to the following reasons:
・If you start the game with a Nintendo Account that has never been linked with a Mario Kart Tour save data file.
・If you have selected the Start Fresh option in the game.
For information on how to transfer your save data, check "How can I continue playing after changing devices?"