I just rewatched The Icarus Factor, and I can't really discern why Riker didn't take the Ares command. Everything we knew about the character up to that point said that his one goal in life was to become a starship captain. Now it's his on a platter and he says no?
The reason he gives, "this is where it's best for me to be right now," is ridiculous. It's not like he's a college junior choosing between an internship and a study abroad program. This is his chance to grab the reins! His own captain pretty much told him, "Take the job, you schmuck!" And still he said no.
I can think of two reasons why he did:
1. He realizes that he doesn't have to compete with his father any more, and he doesn't need to impress him. He finally realizes that he likes being a second-in-command, and that the Enterprise is a pretty good gig. He doesn't need to be an explorer or a diplomat in his own right--he'll just assist Picard.
2. He decided that Deanna Troi was the love of his life, and that a life apart from her, no matter how great it might be, just wasn't worth living. Of course he never told her this, and she didn't figure it out for another ten years, but it's possible.
So they didn't explain why he turned down the first command, let alone the second or third.
After a while, maybe they just started him offering commands as a practical joke.