While obviously an iPod cannot do 3D, there are lots more game titles for the iPod, it has a very good web browser, an excellent camera, and very good sound.
The 3DS can play Nintendo cartridges, obviously, but Nintendo has effectively locked out any third-party content and even their cartridges are region-locked.
Their web browser is terrible and there are very few options for web-based apps, so the 'ebay app' or amazon app are simply not an option.
The 320 x 240 screen is great, but it's no Apple Retina display by a long shot. The sound on any of the DS systems is not great, either. The Nintendo product is clearly more rugged, however, but it's battery life is much less.
For parents, the problem is that it's too expensive. By the time you get the console with all the accessories and some games at $40 each, you're into the $400+ price range.
The original DS Lite sold for around $130, the DSi for around $140, and now suddenly the new model goes for $250?
You can get an iPod touch, have access to thousands of games, most of them costing 99 cents each, and there are LOTS of free titles.