I bought all of them in a span of maybe 2 years. That didn't help.
I rarely use my PSP so I never got through Chains of Olympus due to the big back catalog of unfinished games. And then Ghost of Sparta came out, which may actually be the first I finish.
I'm sure I'll finish GoW III eventually because I'd like to see the ending (not on YouTube) of the story.
Truth is, I just find GoW games in general to be pretty repetitive and very similar to one another. Not that they aren't some of the finest examples of quality crafted games.
If you play segments of all of the games at the same time, most would probably be inclined to agree without obessessing over the minor differences between each.