The problem is that cryptocurrency behaves nothing like a fiat currency. There's a reason putting your entire savings in straight cash is not a good idea: Fiat currency doesn't fluctuate the same way because it's not meant to be an investment vehicle. The main goal of currency is to facilitate value exchange. Crypto is the opposite, by prioritizing value gains over value exchange functionality.
Using crypto to buy goods works OK when the price is relatively stable, but as soon as it makes big movement, lots of vendors stop accepting it. Crypto acts more like (and increasingly moves like) stocks, where investors buy in hoping to profit by converting crypto back into cash the same way stocks are sold.
Which it has done numerous times in its short life. Not a big surprise, and not the real source of the country's problems.
What would default mean in this case anyway? The IMF comes in and claims ownership of state resources?