They are still using the SurfaceDock with the SurfaceConnect Cable: https://www.microsoft.com/en-us/surface/accessories/surface-... , which does basically the same as USB-C. In an era when even Apple is rumored (and even if false, still plausible) to be switching to USB-C for the iPhone, Microsoft still tries to force a proprietary connector. Didn't they learn anything from IE? Even if it's good for them for a while,…
Looks more complicated that it sounds. MSFT already did the job of sourcing cables, connectors, and power supplies for all their surface line for the next XXX years (where XXX is the time they're commited to support all their hardware products). Switching to USB-C from the bussiness perspective would be a great expense... and would involve higher price tag.
They could've started a low cost evolution on a single device instead of handicapping the whole Surface line. Now it has this whole funk of "no USB-C" all over it and it increases the cost of when they upgrade the whole line. And seeing as Surface is experimental (not core to MS, they would shutter it rather defend if it goes south), I doubt that they bought an overwhelming amount of cables, connectors, etc., I think it has more to do with a slower product development cycle due to politics.