1) Tannenbaum had to extend Sanchez in order to create cap space. They had big time need at safety so had to pick up Landry and Bell at a relatively cheap price of $5.4M for the year for both. Further, they couldn't get much more than replacement level players at other need spots (Chaz Schilens).
2) From watching the games, I feel like Sanchez is out there on an island. Running into Moore's ass notwithstanding, his OL has been hit or miss all year, he hasn't had a running game for all but 2-3 games the whole season, his receivers definitely aren't doing him any favors, and he's got Tebow - and the fan base screaming for him - breathing down his neck. Put the offensive issues together with a fading D and you have a real problem. There are a lot of issues with the Jets, Sanchez is not near the top.