Nominally it was because during early pandemic people were worried about handling cash with virons on it, or otherwise bringing infected peoples' bodies nearer to cashiers' bodies than strictly necessary.

Then many realized that their profits didn't take a hit since card payments are common now and nearly everyone has a card.

Now it seems simply to be a convenience thing. Probably businesses are happy with prompting every customer with a tip request, rather than expecting them to remember to drop some money in the jar before they leave with their goods.