Who knows? There could be a lot of reasons, but some possibilities:
1. Spikes is great against he run but a liability in the pass game. He'll get good money based on his run game, but he may be waiting out the offers to get money reserved for a more well rounded player.
2. He left NE on not so great terms. There were lots of rumors that Belichick didn't like how he did things (trained in FL instead of going to voluntary workouts, etc...) and placed him on IR when he could have come back to play in the playoffs. Personality conflicts like that might scare some teams off.
3. While he's good, he's not a superstar so he won't get a bonanza contract. He's probably biding his time waiting to see all the offers before he makes a decision.
A positive for the Bills is that Pepper Johnson apparently really likes him, so we might have an in.