Two reasons come to mind. First, Sony recently rolled out PlayStation Now, which, like OnLive, is a cloud based service that allows subscribers to “rent” games online and have immediate access to them. They were able to eliminate a potential competitor that was already somewhat established in the market.
Second, and this is the more likely reason for the acquisition, Sony gained access of all of OnLive’s patents, as well as their expertise and proprietary technology. Instead of making the same mistakes that OnLive did and having to solve them on their own, Sony can painlessly learn from OnLive’s experience (well, painless except the acquisition cost).