Have you ever taken a bus? Every bus I've ever taken doesn't give change, so it's not a matter of just getting a $20 bill from an ATM and being good to go. You have to have exact change (and often it's not an even dollar amount which is that much more of a pain).

Plus, people paying cash slow down the bus as they fumble for their wallets and count out their coins and such. If enough people pay cash, it slows down the bus significantly. We should be trying to get to the point where nobody pays cash when boarding buses for this reason alone.