Because people are assholes. We don't accept bills over $20 as a matter of safety, because we've been robbed many times and try to have less cash on hand as a result. We have a large sign on the front door that says that we don't accept bills over $20, yet I still get assholes who think we're a bank.
The big concern in the UK is £20 notes. A few years ago there was a huge number of counterfeit ones in circulation, and I still see shopkeepers checking them.
How about people just use their credit/debit cards? There's just no need for cash anymore. I've paid for a single packet of crisps with my card before.
I thought there was a charge for paying for things costing less than £5?
Cash will be a thing of a past very soon with the new 'wave technology' that's in cards now.
Well I prefer cash because then I can see where my money's going without getting a fright when my credit card statement comes in!