No major EU companies want to risk their access to the US and the US Dollar by trading with Iran.
Depends what European companies. The EU companies most dependent to the US are a select few of car companies like BMW. Or perhaps only BMW. Other car companies, like Renault, Citroen Peugeot, don't sell to the US at all. Others don't sell much, like Audi and Volkswagen, both of whom can afford to lose the US market. Fiat? They may probably look to get rid of their US business. Saab, Volvo? US is not an important market to them, compared to say, China.
If Nokia-Siemens and Ericsson built networks on Iran, how can the US sanction these companies, when the US doesn't even have a telecom network equipment maker at all.
What happens if the EU blocks Intel and AMD from getting ASML? What happens if the EU stops buying F-35s, and US missiles like SM-2s? Who loses here more?
Let me add something. Some people saying that the US-China trade war is really a tech war. Maybe they should have looked the other side--- that the EU has already launched a serious tech war against the US--- and doing this in multiple fronts. Read what Article 13 is.