I don't have first-hand experience, but I would guess it would be the same reason most people do: (1) when you roll your own CMS, you have to build all of the features your editors and designers request yourself, the scope of which people tend to underestimate significantly and (2) when you use a hosted solution, you can't customize the site to be exactly the way you want it to be — WordPress offers self-hosting and full customization.

Despite its warts and un-sexy image, WordPress just works for most people's needs when it comes to a simple blog, has a wide range of plugins that solve most anything someone would need from a CMS, can be truly customized, and allows them to move on and spend time on more important things.