If I were going to venture a guess, and this only a guess, I'd theorize that the Rangers thought he had already reached his peak and felt there was only way for his foot speed and production to go. Factoring in a NMC, they probably swallowed hard and assumed that if they were going to make a move, it was either now or never.
The truth is that the Girardi and Staal contracts were disastrous and have impacted A LOT of decisions over the last few years.
This is the sole issue... Those two contracts were $11.2M in cap space, or 15% of the cap... That's an insane amount of cap space wasted on two #6 defenseman... Even if we bought out both of them right now, the cap charges would still account for over 6% of the $75M cap... again, that's only a net positive of 9% in cap space, and it gets smaller as the years go on, and then there's the actual dead cap space factor in the later years of those buyouts.
Those contracts have screwed this team, and that's really on Sather for making such a poor decision to resign both of them when neither of them had won anything. When you look back, they were both no-brainer decisions...
Keeping Stralman over Girardi
Keeping Yandle over Staal
Obviously there is more to it, but the gist of it is that A) If the Rangers had properly evaluated the Defense after the 2014 playoffs, Stralman would be here instead of Girardi (who was already trending down), and B) If the Rangers had waited till the end of the 2015 season to decide about Staal's future here, Yandle would be a more-than-adequate replacement with one year left on his deal, and they could afford to let Staal walk.
Those two mis-decisions screwed this team over. I would even wager that it cost us a cup in 2015.