It is all about control. If you remove the independence of cash and you end up with government regulation deciding how much, what and where you spend your money. Such regulation can be used to put stress on businesses that the state deems 'unsuitable'[1] as well as punish those with differing opinions than the state[2].

Cashless society would essentially turn the world into one giant PayPal.

1 - https://en.wikipedia.org/wiki/Operation_Choke_Point

2 - https://en.wikipedia.org/wiki/IRS_targeting_controversy