I'm not aware of where large cash transactions are banned, but if they are surely it's because cash is largely untraceable. Meanwhile bitcoin is easily traceable, and the IRS is not afraid to crawl the blockchain to piece together what you don't tell them[1]. Putting my tinfoil hat on, I wonder if governments are helping spread this myth that crypto is untraceable because it makes it easier to catch criminals.

[1]: https://www.taxlitigator.com/the-irs-confirms-its-great-eye-...