Why can’t I transfer songs from on old iPod to a new one?

Because the iPod is not designed to allow transfer from one iPod to another.

Instead, if all your music came from the iTunes Store, sign into your account on the new iPod and download the songs from the store.

If any of your music is from other sources, and was therefore added to the iPod from an iTunes Library, you Sync the second iPod with the same iTunes Library that you used for the first iPod.