First, because some random idiot on a blog post does not know whether the person "performed poorly at their job" in general. He just has a specific gripe. The other person might have done miracles in other parts of Cortana. There are such things as shipping priorities, and they're not determined by random blog posts or comments.
Second, because people deserve chances for improving. Anybody at any current position has "performed poorly" at this or another project earlier on. Terminating or transferring them only makes sense if they don't get to improve, which a random blog post can't determine.
>If you are the product manager in charge of Cortana, you failed big time. Again, this should be incontroversial.
That's why companies don't base their decisions on random posts or comments.