The ones still doing cash are deliberately avoiding paying GST, or having a traceable income.
That's a BS narrative sold by politicians and other miscreants that want to be able to pry into every aspect of our lives and be able to control everything, including, eventually, how we spend our own money.
No, it's not.

That's from the lips of tradesmen.
There's a lot here who only work on a cash basis. No records, no GST paid. Cheaper work done.
There's also a lot who will quote for a job properly, and then tell you it can be cheaper if we pay cash.
The government of course wants to stamp this out, which is why there are moves to go to a cashless society. But it won't happen easily, since the cash society here still lives.