Unknown grammar structure

[SebasSnow]

I am working on a paper and I came across the following sentence: "it is a perennial puzzle why no woman wrote a word of that extraordinary literature when every other man, it seemed, was capable of song or sonnet" I know it is used for emphasis and I believe it is a case of fronting, in which preparatory "It" is needed, but I'm not sure. I also thought it could be a cleft sentence, but I have not seen any case of cleft sentences with "why". Does anyone know what kind of structure this is? Thanks!

[Alan]

It is a cleft sentence!