My basic answer is : I stopped playing because I was bored while playing it.
I happen to play Civ since the first one and I remember how the process to discover, understand and figure out strategies was a long process. And it made half the fun, lots of parameter, understanding how they interact etc, accompanied by the slow progress of difficulty levels.
On Civ 5 I achieved a 1000 AD Riflemen (or whatever they call it) rush with such ease on my second game! And I'm not an elite Civ4 player, at the best I can win on emperor with a good start.
I didn't feel like I had many difficult choices to make and learn from...
I also think the 1upt is a disaster, it tries to get a tactical part to the game while being wildly clumsy and it has a lot of consequences on the whole game. I recommand you read Sulla's final critic about Civ5, it explains how the 1 unit per tile choice has consequences for the whole game and not just the war system :
http://www.garath.net/Sullla/Civ5/whatwentwrong.html
I don't play Civ4 regularly but I do come back now and then and I still find articles interesting and new strategies and I read some posts and say "well thought" and I'm eager to go again and try to adapt and improve.
In Civ5 I think the solutions they took to avoid issues are killing the game rather than upping it.
You need to find a reason to stop a sprawling empire to just steamroll everything, yet the global unhappiness discourages empire building, and you end up with an empire of villages... Not elegant, not fun.
You need to make diplomacy more interesting than just piling +/-, yet making every leader a psychotic irrationnal unreadable person does not grant depth but randomness.
You want to avoid huge piles of unit, etc (a piling limit linked to tech, road and terrain could have solved the problem for example).
Yet I realize most of my post says the same than the first sentence : I get bored and it's hard to pinpoint a rationnal argument about what makes it so...