You are missing some points:
1) Civilization can be played only by Steam, the other games, like EVE (someone uses this example) don't use Steam. And even if they are only the online players and are there are others playing offline, it is still indicative for a trend.
Yes, a trend that you will likely observe with every single video game. Except for the people who become hopelessly addicted to a game (which isn't a good thing) it's quite normal for people to eventually move on or at least play a game less frequently than when they had just bought it.
Really, this sort of trend is not some revelation.
2) It is constantly dropping, but it has a big difference with games like New Vegas, Civilization has a replayable value that some other games like New Vegas lack, so it's worrying that it has dropped so much in players. And i speak of a game dropping players so much only after two months from release...
Two months after release is not a very long time. I'm not playing civ5 lately but it's not because I've given up on it. I suspect a lot of people are doing the same. We're simply waiting for fixes/additions to be made to the game first. When a game is farily buggy at release (or when the first one or two patches introduce bugs

So that's the point. You need to understand that Civiliation use Steam as a base to play, when other games not. It's a crucial evidence...
Evidence for what? That civ5 is a less popular game than other mass market games like Call of Duty or Football Manager?
I'm struggling to see what actual point you're trying to make here apart from the obvious trends that happen for every game.