I would very much like a real explanation for this question from some expert in virology. But I just assume in some kind of similar way to how measles came back in Canada among the antivax community in recent years.

They are anti-vaxxers, yes, so no wonder that they could technically get measles, but where did it come from? It was completely irradicated because most people had the immunity. The virus should have died off and disappeared from the face of the planet. And yet here were are. There must mechanisms for viruses to survive this.