why not? Why do you want different USB connectors? To be able to buy adapters?
Actually it appears the other way around to me.
There's not a lot of "stuff" that's USB-C yet and there probably won't be for a little while. Except for data transfer of large files (and of course, the fact that it's reversible); there's not a tremendous advantage to the blazing speeds of USB 3.1 and TB3, over the speeds of USB 3.0.
Even Apple recommends you connect fast external storage directly to the MacBook Pro so that you can take advantage of the full speed of USB-C, without bandwidth being hogged by the display. So, folks aren't going to be connecting large storage array's directly to the display. They
can, of course. But best performance comes from not doing so.
So that begs the question; what is actually going to be plugged into the display? Slower storage mediums or storage mediums where a lot of speed isn't really necessary (like an SD card reader; which can't saturate USB 3.0; much less USB 3.1 or TB3), things like USB powered mice and keyboards, game controllers, smartphones and tablets, USB headsets, peripherals like DJ or photo/video editing "controllers" (for lack of a better term; USB boxes with buttons and dials on them essentially). Much of this will be already owned by folks and even if you buy it right now today; it's almost certainly USB type A (generally USB 2.0 or 3.0).
So while the USB-C connector to connect it to the MacBook Pro makes perfect sense; the 21.5" model which only supports USB 2.0 transfer speeds having USB-C connectors doesn't make much sense. Because nobody is going to plug in blazing fast USB C stuff. And even on the 27", the fastest storage is going to be direct to the MBP.
It's not unprecedented...
Fair enough! And, well, I'll admit maybe "never say never", I just really don't think so.
Especially because Apple
used to sell these products. Apple used to make Printers and Scanners (really good ones at that); when Steve Jobs returned to Apple he axed all of the extra accessories and things in favor of focusing on the Mac and other consumer devices (namely, the iPod). In the Apple Stores, you could buy printers and scanners from other companies; and Apple has never gone back to producing those products. Granted; few people USE those products anymore anyway.
Point being; while they've pitched another companies product and then produced their own version later; I'm not aware of them selling a product, canceling it, pitching an alternative from a former competitor, and then later returning to the market.