Because there is very little evidence that the company can reproduce this success, or even that the current rate of revenue for that game will persist for any meaningful period of time. It is pretty clear that they (Zynga) made a snap judgment and decided to pick up the company, regardless of the cost.

This reminds me a lot of when Mark Cuban sold Broadcast.com to Yahoo. Yahoo execs overreacted to the potential of the technology that Broadcast had developed, and were drastically overpaying out of fear of missing out. Not a great position to make acquisitions from.