They weren't. Diamond and Pearl introduced many new features and positive changes.
Also, they have the largest number of obtainable Pokémon in a single game so far.
While there were a large number obtainable in game most of them were after the elite 4 so you couldn't use them to play through the game. Before the elite 4 the number was 150 the least amount since Gen 1 (which was excusable as there were only 150 in existance at the time).