no reason besides his health
Pretty much nailed it right there, didn't you?
At this point, Oden's best-case scenario is that he can be an effective, short-minutes backup. People need to forget that he's GREG ODEN, #2 pick that was expected to be a dominant force. He's a guy that has obviously had severe injury issues, and not just freak injuries - stuff that he's never going to recover 100% from.
And, we've got plenty of backup centers right now. Vitor's a backup. KO will probably play somewhat at the 5, just to stay on the floor. Zeller's a guy that you'd like to have coming off the bench, not be your starter. Even Iverson might be there as a third-stringer.
One of those guys will be a "starter," but only by default. We don't really need another.
Although, I suppose he, Evan Turner, and Jeff Green could commiserate about their statuses as former top-5 draft picks whose careers have been stark disappointments.