That's not even a malicious choice, either. Hype influences our experiences and perceptions of whatever is being hyped. It's intuitively obvious that people will enjoy a good thing that they are hyped about more than a good thing that they are not hyped about. Hype is strongest just before release... which is exactly when reviewers play and assign a score to a game.

A sequel to a well received game is going to have more hype than the predecessor in most circumstances. Morrowind sold something like 5-10x the copies as Daggerfall and came about at a time when there was a lot of upheaval in the industry from a target-audience standpoint: a lot of potential Morrowind players (and reviewers) would have not played Daggerfall.

In essence, Oblivion was reviewed more positively because of the positive reception of Morrowind. The positive reception of Oblivion in turn boosted Skyrim.

This is not to say people would hate the games without the prior game before it or hype, just that there is a "hype boost" for games.