Thunderbolt is not a proprietary connector to Apple. It is a standard that Intel has made available and i've seen non-Apple computers with Thunderbolt.
And to add to this, Intel originally debuted Light Peak (see "Early Versions of Thunderbolt") with a USB connector, not the current connectors you see now. Unfortunately, the USB consortium said "Nope, you can't combine the two." So Intel re-released Light Peak as Thunderbolt with the new open standard.
Probably because they had USB-C in the pipe.