Because people really just want Civ IV: BTS part 2.
A new Civ game only comes out so often. I WANT them to start fresh, start with the basics. Then build upon that with expansion packs. Civ IV: BTS was nowhere near basic, and personally, I didn't care about espionage and corporations. The expansions are there to add things, and should be considered optional extras. So, start with the basics, make them work right, then add new features and complicate things through the expansion packs. If thing's start out already overly complicated, two things could happen:
People who like the more simple approach will be elimenated from the sales. There are vast amounts of people that hated how complicated Civ IV became. I know people love Civ V but couldn't really get into Civ IV because of it's complexity.
people could be thrown off, because it would take ages to learn the game. Even with Civ IV, it's easier to start out with Vanilla, learn the basics, then move on to more advanced stuff in BTS. With a basic Civ, especially one that is as drastically different from all others like this (5), we get some time to learn the game properly, then when it's been patched up, tweaked and stuff like that, a proper expansion pack that adds new elements and features will be easier to enjoy.
Also, if Civ V was Civ IV but with 1UPT and hexes, people could call it a rip-off and wonder why the hell not just play Civ IV.
Which is what I ask of you people who are so desperately clinging to Civ IV:BTS - why don't you just play that if that's all you want? What exactly did you want from a new Civ game?
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