I think it's because other countries or big companies don't do their business in BitCoins.
A currency, whether it's a fiat currency or a digital currency, is valuable as long as you can use it to purchase the goods you need. The value of the US dollar, regardless of the US relative economic decline in the last decades, comes from the fact that the US maintains strong control and influence over the energy and technology markets in the world, mainly thanks to the petrodollar agreement and Silicon Valley.
Now Iran herself doesn't need oil. But the question is, why should other countries, particularly Iran's economic partners or countries that export to Iran, which happen to need oil, accept BitCoins when they can't import what they need with BitCoin?
However, BitCoin or other digital currencies can be used indirectly for money laundering by a network of intermediary companies to import dual use and sanctioned goods to Iran. But it won't be of much help to Iran for conducting business directly.