Some people think S. T. Joshi is a brilliant scholar.
The fact is that literary criticism is a field that welcomes con men.
I think S. T. Joshi is a persuasive con man with the intelligence typical of card sharps and insurance fraudsters.
Lovecraft is popular with a lot of people for the wrong reasons. A lot of folks who should take a serious interest in paranormal topics read Lovecraft instead and laugh off all paranormal issues as pure fiction.
Thus influential atheists tend to promote Lovecraft to mass-market audiences, and con men like S. T. Joshi spot the suckers and fleece them.