[crazygringo]

The article clearly explains that the link isn't clear at all.

It's that certain damaging proteins are a line of defense against the HSV1 virus, that something sometimes sends those proteins into overdrive, that this is influenced by genetics broadly, further influenced by a particular gene, and that it's a second infection with shingles that can reactivate the proteins, worsening it.

Given that this is the interplay of something like at least 5 factors, and there may be more, it's not surprising it's taken this long to put together, even with all our statistical tools.