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WordPress is older and more entrenched. When you have knowledge and infrastructure built up around a certain solution, it can be very difficult to switch to an alternative, even if the alternative might be intrinsically better. The cost of switching over disincentivizes switching over. Most static site generators that exist are also oriented towards developers. Not everybody is a developer. A lot of "normies" just want pretty-looking pages. Moreover, "WordPress" is a specific, branded product. "Static site generators" are a general technical methodology. Brands and products are more marketable to normies.