Always chuckle at how something that is explicitly not part of the game, fighting, as there is even a larger than normal penalty for it, is what fuels rivalries. It's not the game of hockey that does it. It's the extra stuff. Football is very boring to watch, but gambling on it is so easy. Why do Flyers fans hate Crosby? Because he's good at hockey? No, it's because he'll hit the glove away from an opposing player trying to pick it up.
Sports used to be the last place you could actively hate and it was ok, because ultimately, the result of the games aren't do or die. The Lakers and Celtics physically went at each other. The Bad Boy Pistons took it to Jordan, and anyone they played. The Riley led Knicks would knock you around all over the place. Today, players are increasingly friends, and in the NBA they go so far as to plan years in advance as to where they will play, and who they will play with.
Free agency was mentioned, and I definitely think that's part of it. When players had less power and say over their careers, it seems sports were better. Today, everyone knows it's all a business. A high paying business at that. We as fans have always been emotionally, and irrationally, attached to corporate logo's, but now we also have to care about salary caps, and contracts, and big and small markets and what's fair or not. Gretzky gets sold, Stevens ends up as compensation for Shanahan, Lindros chooses not to play in Quebec, players vs. owners is more contentious than what we see between player vs. player, fans end up being stuck watching players that have contracts that don't match their production, and knowing all too well how much players get paid. These are all part of the extra stuff outside of the actual game of hockey, but it's far more boring and detached than watching two guys, or twelve guys, or even more, hitting each other in the face.
We want blood. We love blood. We don't want technically proficient hockey. We don't care about the personal lives of players that fans will never be friends with. We want physical violence. At the same time, we'll cry about cheap shots, even though we really do want the cheap shots. We want the elbows to the back of the head, the stick swinging, the revenge.