Acken
Deity
A vast majority of Civ players don't play on anything near Apollo/Deity difficulty. so I doubt it's cost effective for them to focus resources on such a small percentile of players who do. In fact I'm betting they probably toned down the difficulty to cater to players who think Civ5 Deity is too hard. People like feeling like they're beating games. Do we really need 50 threads about this? Vocal minority is vocal.
These players would never skip a game because they thought deity was too hard in the previous game that's non sense. Nor is imagining that somehow making them win easily in the next game will somehow boost their sales. Players stuck on a level are usually satisfied with the difficulty and will blame whatever for not trying to beat the next but I fail to imagine their customer decision is based on it.
What makes sense is to not devote too much resources yes. However, top players are still a part of your customer base so while not devoting all of the resources to them is obvious, devoting none would be as stupid. Also, yes it's a vocal minority so what ? Core fans are and always will be more vocal because they are your fans by definition and will represent the main portion of people making your game live after the initial release. They are also those making most of the reputation of a game years later. Pissing them off is not necessarily such a smart move in the long run.