Completely different market dynamics.

Consumers don't all want to own the same smart phone, in fact quite the opposite: consumers are completely unwilling to all own the same smart phone.

Then to top it off, the smart phone market is roughly 20 to 30 times larger than the iPod market. Which means a helluva lot more competition. There's also no simple blocking method available to Apple to own the smart phone market, unlike the iTunes + music licensing moat they had in music players.