Till theft, the physical act of counting it and securely storing it, screening counterfeits, securely transporting it to the bank, etc. I don’t have direct links handy, but there have been some good insightful comments on the topic on this site previously. Hopefully they will turn up without much difficulty with a search. Sorry to have to pass the buck on a more detailed answer.
Handling cash is very labor intensive, you're paying your highest paid employees (managers) to count cash multiple times before/after shifts, bank runs to deposit, making change, rolling coins, etc. Plus the cost of physical security for that cash.
If you handle enough of it, you probably wind up contracting with an armored car company to come collect it. Some gets lost or stolen, coins have to be rolled up for deposit, etc.