Because if anyone has ever thought that Thunderbolt would be used for all accessories they were very, very, very wrong. You wouldn't plug in a printer, or a flash drive, directly to a Pciexpress bus, would you? There's simply no need. And yet, this is pretty much exactly what Thunderbolt is. It's fantastic for things which need this kind of connector - external graphics cards, 10Gb optical network cards, RAID controllers - but not pendrives.