As other person pointed out, mutual funds are much more expensive.
http://www.ft.com/cms/s/0/3e0cc962-ec0c-11e4-b428-00144feab7...
As for it only "technically" being a mutual fund: consider that VFIAX predates VOO by 10 years. How does that square with VFIAX only "technically" being a mutual fund, when presumably it was a real, honest mutual fund in the years 2000-2010 (before VOO was introduced)