merchants want it because it's stable.
Analogy: Can't stop people from using gold and silver for payment aswell. Some just store the bullion but others actually use them as money.
"Store of wealth" which is flexible and portable (which crypto is like nothing else) can be used as money too.
It's ment to store value but because it does that it actually has a certain value and so you can use it to pay someone else with it
who accepts it.
"Store of wealth" and "currency" have a relation with each other.
I can pay you for Kim Jong Un's haircut in Urea if you would be willing to accept urea as a payment (or cement, gold, silver, corn, cats, dogs, goats, bread) .
You accept my 5 cows and 3 goats as payment for your daughter and the deal is done (this is actually how it goes down in some countries). Goats and cows can be money or store of wealth too.
(just an example, not wanting to buy your daughters with goats
)
Saying: if it has value / stores value and is easily portable, and different parties in a trade recognise this in a similar way it can be money - no matter what it is. You only need: someone to accept it, an agreement on its value and it needs to be portable - so there is your theory of money
if it's no store of value it's not money - on the other hand