Stepan was a good player, let's not discount his contributions.
While he never quite became that 25 goal, 70 point player he appeared to be morphing into 4 years ago, he's been a fairly consistent year-to-year producer: 17-22 goals, 53-60 some odds points (if healthy), and he's never been a minus in his career.
But we live in a salary capped NHL world and like Derek Brassard before him, it was a question of money, contract and anticipated production over the course of the contract.
I think the Rangers were probably expecting him to be more of a driving force on the ice, and he was once again a support player. There's nothing wrong with that per say, I just think it became apparent to the Rangers that now was the time to make a move if they were going to make one (which especially makes sense because of the NMC).