From the Developer's web site:

Exerpts from the FAQ:

The Settlers: Heritage of Kings still is very much about building up an economy. Affectionately made animations and a living environment have consequently been a part of the series. The player still can just sit back during the game and watch the busy little settlers doing their work.

It was important to us that the Settlers Saga evolves further. Therefore there have been several changes since The Settlers 3 and 4 including the new graphics style, the 3D environment, a storyline and the heroes.

That is also the reason why we do not speak of The Settlers 5 but of the Heritage of Kings.