I can guess why that might be.
(I wasn't thinking bank charges. Businesses get charged for banking cash too.)
I didn't really mean tradesmen, I meant shopping and retail. My sister is a hairdresser and she still get's paid in cash, but the problem today is where to bank all the cash. We've just received letters this week that two more local bank branches are closing. The nearest branch is now 5 miles away. I live in suburban London. If you live somewhere more rural then that could be 20 or 30 miles away. And keeping piles of cash hidden about the house isn't a great idea from a security standpoint; even less so if you drive a lettered van and the criminals can see you might be someone who takes large amounts of cash.