That could be part of the reason, but also I think insisting on chip all this time just really slows down a store that doesn't really spend on cashiering all that much. Tap to pay can shave off a few seconds on every transaction (ever been behind someone in the store who finally pulls out their wallet, finds the business end of their card, inserts it, waits 20 seconds...).

Millions of transactions a day, over thousands of stores, it can make a huge difference in time saved.