The funny thing is people still think there’s some “mechanism” or piece of paper that will prevent Iran from wanting and eventually getting nukes.
JCPOA as it originally functioned could make a credible claim to be just such thing - all the objective evidence showed that it was working. Even US government agencies openly stated that Iran was complying, only the Trump admin judged it was not enough (without ever spelling out what "enough" entailed in their view). Now that US withdrawal has brought the treaty to collapse, the very notion has been dealt a severe and lasting blow, of course. It has called US reliability as a partner into serious question, and with it the worth of entering into similar agreements in future.
You have to hand it to John Bolton - between abrogating ABM (the resulting developments demonstrating that US informal assurances are useless) and exiting JCPOA (showing that even formal agreements cannot be trusted), it's hard to see how he could, essentially single-handedly, have done even more damage than he in fact did.
So. . .what, Europe can't be in the JCPOA if the US isn't? Why didn't Europe negotiate an agreement of their own?
Europe was party to JCPOA (which is technically still in force) from the start and remained so after the US withdrew, trying its damnedest to keep the agreement alive and hence Iran's nuclear programme contained. However, as totoro has pointed out already, US sanctions were structured in a way that sabotaged any such attempts, because they effectively hold European companies' economic livelihoods hostage. By threatening penalties on its activities in the US against any entity doing business with Iran, they scuppered European efforts to uphold the Western end of the JCPOA deal.
If you force a typical private enterprise to chose one or the other between serving the US or the Iranian market there can be only one outcome. So although the European (and, for a long time, Iranian) governments were willing to continue honouring the deal, the inability of businesses to cooperate eventually rendered it untenable. Not condoning the Iranian escalatory moves which followed, but they don't alter the clear cause and effect relationship.