The primary reason seems to be “stagnation” in the number of hits provided by StumbleUpon. This is despite the fact that adoption of the plug-in has continued at a steady rate, currently installed on an estimated 6 million browsers. That's quite a lot of people to have use your plug-in, though clearly eBay doesn't feel it was worth the $75 million they paid. They may end up selling it at a loss, or even rescinding the sale offer if it doesn't go as they hoped.
StumbleUpon made a huge leap about two years ago, when they first offered multi-browser support.