Well, for one, the reason is very obvious. iPhone users got the upgrade for FREE while iPod touch users had to pay several dollars to get the upgrade. That alone is a big factor.
Setting aside the price difference though, Chitika also attributes the unwillingness of iPod users in upgrading their OS because it has little to offer.
“Push notifications? MMS? Tethering? Essentially useless on a device that relies on WiFi for a connection. iPod touch users are essentially asked to pay for copy/paste, in-app purchases, and the ability to buy a segment of the latest apps from the app store.” Chitika says.
The upgrades that can be applied to iPod touch users only include cut/copy/paste, in-app purchases, and the ability to buy a segment of the latest apps from the app store. Those who wanted to have the upgrade (legally ;)) had to pay $5 to $10, depending on timing.