It's because the .COM is well known by Matt and Ryan Boren. It was out before the .ORG was. PLUS, .COM is a major free blog hosting software, where as .ORG is for those that want to download the software and host their own blogs and get support from .ORG.

.COM = free blog hosted, free support, limited usage (themes, plugins, etc)
.ORG = free software, free support, plugins, themes, and you can host yourself, and use unlimited themes, plugins, and what-not.

The .COM uses their MU Software, "multiple user". For multiple sign-ups, and can have your own blog there free of course, just you have limited usage of themes they select and so forth. So, SEO wise... the .COM would be good choice, however, if you host your own, you could also have good SEO with that route as well.

I remember making posts on my own personal (self-hosted blog) and few days later, it came up between 1 and 3 pages of google.com. WordPress software, is good for SEO regardless where it is actually.