I've seen three different theories on this now, and I think I disagree with all of them:
1) Your theory (and I realize you're partly just being funny) about encouraging iPad upgrades by not offering as much protection. I doubt Apple would even think that's effective. People would just do what they're doing now--get their cases from third parties.
2) There's a heat issue that Apple is trying not to exacerbate by selling cases. I doubt that's true as well. My 9.7" Pro never broke a sweat and never got more than slightly warm to the touch, even after a lot of more resource intensive use. My 10.5" Pro is the same way. Also we must keep in mind that the 2017 standard iPad model also didn't get a back cover for unexplained reasons, and I doubt that model has heat issues either. Also, if there were any widespread heat issues, we'd definitely know by now. Think back to how fast that story spread when the "new iPad" (iPad 3) came out.
3) They're trying to push the leather sleeves. I don't think that's it either, for the same reason I listed in #2--the standard 2017 iPad isn't technically compatible. I suppose you could buy one for that model, but it wouldn't fit right, and you'd have a useless Pencil holder at the top for an accessory that doesn't work with your iPad.
I think the reason is probably just that the back covers don't sell very well. As awesome as they are, and I do love using them because they fit so perfectly, I never see anyone else using them other than myself. Most people seem to buy cheap third party cases. It's a shame though--I was VERY fond of my silicon back covers for my 9.7" Pro. I had a gray one and a blue one and loved both. The blue one looked really cool against my gray ASK as well as my lighter blue Smart Cover.