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)
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)