iroh went with zuko because after lu ten died iroh cared for zuko during his banishment like his own son (he said it in some episode i'm not sure which one) as you mentioned before.
also iroh wasnt that respected after he surrendered at Ba Sing Se, i doubt many respected him considering ozai probably named him a traitor
also iroh probably wouldnt have chased after the throne because he probably didnt get word until ozai was in power. also iroh was probably depressed after the loss of his son to do anything about ozai