US drug control agency will move to reclassify marijuana

One major effect of this is that weed stores will be able to use banks and payment processors legally once the regulators catch up.

I don’t get why they don’t just use Bitcoin or Ethereum.

Mainly because your employees, suppliers, and landlord have no desire to be paid in Bitcoin.

What does that matter? You can convert Bitcoin into cash. But then you can do that in an undisclosed location at your leisure instead of keeping a mountain of cash in your publicly advertised storefront location and becoming a huge robbery target.

Also, it doesn't seem like it would be that hard to find someone to accept it as payment, since they can convert it to cash too. And the sort of landlords/suppliers/employees willing to do business with a dispensary seem like exactly the sort who would accept Bitcoin.

So if I as a consumer want to buy weed:

I show up and convert cash to bitcoin, presumably losing some of its value to exchange fees.

Pay the merchant my bitcoin, who then has to convert it back to cash losing more of its value to fees so that he can pay all of his staff, suppliers, utilities, etc...

Why not just skip the bitcoin step and save time and money?

Because you don't have cash to begin with, you have money in a bank account, which you have to convert to something else to pay the dispensary. Converting it via cash instead of Bitcoin just makes it easier for you to be mugged.