[ars]

It might be rare. If it lasted say 100 years, a supernova could make some, but it wouldn't last long, making it quite rare. (Depending on how you define "rare", is rare a relative measure compared to other elements, or an absolute total quantity?)

> the conditions in our accelerators are singular in the universe.

Most likely they are. A supernova is a random event, an accelerator is directed. It's quite possible we could make lots of something in an accelerator that would never happen in the random conditions of a supernova (for example if some other more common effect consumed the raw material needed for the rare thing to happen).