This feature is similar to many top cooking games where coins are specific to a restaurant or city and don’t carry over to the next one.
We do this to keep the game challenging and fun, as there are many ways to keep earning coins (like watching rewarded videos, competing in events, playing new levels, using boosts, etc.) which would make it too easy for many players to upgrade future restaurants.
Also, know that we are working on new features that would let you use your old coins, so stay tuned for these while you continue to enjoy unlocking new restaurants!