You go to the countryside, or on vacation, or you just need to kill 8-10 hours and you don't have internet access, and how many people won't be able to play simply because they don't have internet!
And I would say that Ubisoft's policy isn't great; I even think they're counting on some degree of failure with the 6th installment, but on success with the expansions. They're focusing on the European and post-Soviet audience. And they introduced 2 new races: South American Papuans and Naga-Asians, and the Naga are a trump card that will be enjoyable for anyone to play.
Well, if the budget doesn't pay off, then dragons, elves, and dwarves come into play – the heavy artillery of the Heroes series. And it seems they're focusing on uniting fans of Heroes 3 and Heroes 5. In short, the guys decided to make a killing.