I'm not going to lie, I've "wondered" this myself.
I remember in that E60 or whatever it was on him mentioning that when Katrina hit, a fire was lit under him. He hated seeing his family in that situation and he wanted to get them out, and that way out was football. Well, he got them out, he bought them a dream house recently, so that "dream" or "goal" was realized.
I'm not saying this is his one and only drive in football, and all he wants out of the game because frankly I don't know Eddie Lacy, so I won't pretend to. I could be totally off base with this thinking, but to me, he comes across as seeming "uninterested" at times. Even when they benched him in favor for Starks, his response seemed very much, "Eh, it is what it is. Will it get better? I dunno, we'll see... Starks is better this year, I don't blame the team...we'll see what happens..."
I like Lacy, a lot actually. And wish he could get it together, but like I said, he just seems uninterested at times.... but then again, maybe it's just his personality, maybe he's just that chill...