The prolonged low interest rate environment may also have pushed up the demand for cash. As for the supply of cash payments, cash payment networks supported by financial institutions' bricks and mortar branches and ATMs are important (Figure 4). The denser the  3  networks, the more convenient cash withdrawal from the bank account and cash deposit to the account.
For this reason, the greater the number of branches of financial institutions per habitable area, the higher the ratio of cash in circulation to nominal GDP tends to be.