Its a big rivalry because it is usually for the AFC title, or has big playoff implications. Not to mention, that there was a whole NFL rule change (5 yard rule for CB's) because of this rivalry (thanks to New England being a bunch of lowlife cheaters) It always sparks a hot debate between which team has the best quarterback.
Also, I may be wrong, but I do think that they hold the record for the largest national audience in a regular season game