Probably a question of businesses who need 10gbe are willing to pay for it, while home users don't really NEED it, so the prices are artificially high. Meanwhile USB is a bit more of a consumer thing. I recall even 1 gig took a while to become affordable. Even now a new 1gig 24 port managed switch is going to run you close to a grand. Though you can get them on the used market for ~$100 range.
I think 10gbe will start to come down over time though. You can buy gig switches at a half decent price that have like 4 10gbe "uplink" ports, so that's a start.
Think it will be a while till we see it on consumer motherboards though. Heck even existing gig on consumer motherboards is often sub par. I personally never had issues but heard lot of network gurus say you're better off putting in an Intel nic.