Probably a chicken-egg thing. I, for one, don't have a single USB-C device at home. Zero. Nothing that can plug into a USB-C port without an adapter. No plans to buy anything that does, unless I'm forced to by computer/phone manufacturers. I would be... unenthusiastic about a motherboard that emphasized USB-C over other ports. Includes it, so I have the option to start transitioning? Sure.
Work's similar. There are a couple USB-C Android phones floating around, but the other end of their cables is—drum roll—USB-A.
I think that's my point. PC vendors are slow to change. Not to worry, 2017 will be the breakout year for usb-c. Several hundred million phones will ship with it:
PC vendors can't afford to tell users to throw away all of their accessories every time a sexier connector comes along. Some other vendor will include a ps/2 port or a parallel port or USB type A and if buyers want to keep using the stuff they have, they'll buy the new computer that has the ports they want. Apple can do whatever because if you buy the old model for better ports, you're still buying from them.
Absolutely no one said that they need to replace all their legacy ports. Adding a couple of the small USB-C ports to the front and back is all that's needed on desktops. On laptops, putting one on each side should be doable.
A computer that you buy today will last 7-10 years. I guarantee that there will be lots of USB-C devices in 3 years.
Work's similar. There are a couple USB-C Android phones floating around, but the other end of their cables is—drum roll—USB-A.