TL;DR: price, simplicity, performance
SmartVideo offers unlimited videos and bandwidth. Wistia does not.
Wistia is great for a/b testing of multiple videos while creating landing pages for PPC campaigns, but it's overly complicated and pricey when simply trying to serve videos on your website.
A Wistia account with access to an unbranded video player starts at $99/mo and includes only 10 videos.