Its business.
Its long.
He was set to get 6 million (or so) on his last year. He had already hinted that he might holdout if not extended. The Steelers are up against the cap, but have everyone (60 players) signed for this year. They had about 4.9 free, but needed rookies and just resigned Aaron Smith.
Then you have Porter having a bad year and what could be a decline and he wants more then 6 mill he is on tap for. If anything the team may have asked him to renegotiate to take a cut, but that isnt what he ever said he wanted.
Now if you remember Jason Gildon, he got the big contract at about the same point and it was a terrible move. So they may be thinking it easier to replace (eventually) then to pay out big bucks on a guy on the wrong end of his career.
Add to all this that
Silverback is waiting in the wings... James Harrison.... and this move becomes real easy to swallow. Many Steeler fans (and maybe coaches) wanted to find a way to get him on the field. He has shown great ability in limited time. But the Steelers have been neck deep in linebackers. While he may never be Joey Porter, its not as if they have to scramble for a replacement.
Then... you have Tomlin. Porter was ALWAYS Cowhers guy. Well, there is a new sheriff in town. And that combined with the fact that Lebeau has already mentioned the possibility of using some 4-3 at times, and the money they would have to invest in Porter just becomes way too much to consider.
Steeler fans will miss Joey. Thats for damn sure. He can "bring it" for someone else the next couple years. But in the mean time the Steelers will be able to resign Smith and Polamalu and Roethlisberger.
The Steeler will also once again be able to use the draft at a position that they absolutely own when it comes to drafting - linebackers. Where before it would have been overkill to look at OLB.

Are you still awake?