I'm pretty sure the major reason MySQL is "more popular" has more to do with its ubiquitous install base on low-end web hosting providers than the (non-)existen

I'm pretty sure the major reason MySQL is "more popular" has more to do with its ubiquitous install base on low-end web hosting providers than the (non-)existence of Windows-based GUI clients — particularly given that PgAdmin III has been around for at least 10 years, and cross-platform the whole time.