Future expansions.

It's easier to create 5 Factions and 24 maps than 8 and 36. And then, expansions with 1 new Faction and several new campaigns will be sold for the same price.

If the developers included a huge number of characters, units, and Castles (8, for example) in each game in the series, whether it's the 3rd, 5th, or 6th installment, then the plot wouldn't matter. Nor would the graphics. And, in addition to all that, if the distribution of powers and magic among classes wasn't the same for everyone, as it is in the 6th game, but rather diverse, then everyone would love this pseudo-series of games. People like massiveness and variety, but what do the developers do? They create an initial version with limited resources, abilities, Castles, and units. And to prevent the game from "dying down" in the eyes of its fans, they develop expansions. This raises the question: why can't they just create the expansions together with the game from the start, and then visualize the game and add small, but effective "features"? It seems that this is a financial aspect that we cannot understand.