This is an older article, but one that has really stuck with me since I read it.

D&D had its origin as a dungeon crawler, in which the majority of the game takes the form of entering another room and encountering a new mystery or obstacle that is supposed to be different from what they solved before. That's a kind of game that benefits from having a lot of monsters to pick from.

But over time, the basic premise of what new editions of D&D changed, as were the premises of new games coming out. The concept of "high level" play also changed, and eventually we reached the point where the cool, imaginative boss monsters moved outside the scope that most campaigns would ever reach.

Joseph Manola made a very good observation of why campaigns should use unique boss monsters at a much lower power range than they are usually statted for in many contemporary games.