It will be ready later, now they're busy on other things.
http://www.gamespot.com/pax/xbox-one-wont-support-external-storage-at-launch-6413955/
Who needs external storage for a console that's also a home theatre multimedia device with strong focus on digital download, but with non-replaceable HDD, after all?
But thinking again about it, this is coherent with MS cherished tradition of launching incomplete products, that dates from before Windows 95 and has become a sort of amulet that cannot be renounced, so it's NOT a missing feature, IT'S A FEATURE!!!