Good luck plugging a USB key into your iPad, or letting your security-sensitive workplace let you plug arbitrary USB keys into your workstation, or convincing your bank that you really did not send your entire balance to Nigeria, even though you signed that transaction with a tap, etc etc...
Remember Mt.Gox? That's Yubico's most public failure so far
Strong authentication needs to be out-of-band, and support transaction signing, and work everywhere, or there's no point using it. You can't get "out of band" with anything that you "plug in" - that's simply connecting it directly to the same threats.