They (Iran) obviously chose it because it is at least in the short term comparatively difficult to sanction but also not to difficult to convert. It would not do them much good to collect tolls in some other currency and subsequently have US diplomatic pressure cut them off from the banking networks that generally handle that currency.
I am not saying that Bitcoin isn't highly traceable and that the US and other governments wont try and won't ultimately succeed making it so punitive to accept payments (in bitcoin) from Iran that Iran has to trade their bitcoin at a significant discount to those wiling to launder it and / or accept having tainted funds they can't spend a lot of place as well. That will happen, but will take some time to become really effective.
It does leave you wonder if the people who whisper in Trump's and Bibb's ears and their friends who might whisper in the ears IRGC types might indeed have provoked this whole thing in order to create a situation Bitcoin or some other crypto-currency could be forcibly inserted into the "international system".
It is interesting to think about because I do still believe in light of hyper-sonic missile and drone tech, and expanding Chinese influence there were / are good strategic reasons to remove Iran as an international player right now, for the US. Attacking Iran was a smart move... Letting them survive as even a regional power isn't. The ability to fight them is determined by the domestic clock on war powers. Trump is an idiot for wasting two weeks on this cease fire, he should economically disabled them, finishing it. Whoever he listened to on accepting those terms is not advising him well. We should have at least destroyed their remaining oil infrastructure, before any pause.