The main factor could be that there is no other successful CMS + web server engine _at that scale_ out there. There's plenty of other options if you have the mo

The main factor could be that there is no other successful CMS + web server engine _at that scale_ out there. There's plenty of other options if you have the money or the skills, but both of theses are eternally in low supply. Abusing a blog engine to make it run small EC sites becomes a cheap and half manageable route. Even for super simple things like a site with a presentation page and a contact form, a wordpress blog is faster to set up, cheaper and more manageable and ultimately more reputable than many solution out there (the "cheaper" part being of course the more critical aspect)