Cynically, you answered your own question: $80 is why it wasn't included. Apple is one of the best companies in the world at convincing their customers that a product they were satisfied with only yesterday, is somehow inadequate today. Not every company can do that. Even those that can, can't do it like Apple. You'll even find customers who will claim it's a defensible decision... and they will defend it.
They could've offer it as a BTO and offer a discount for those who doesn't want the slower ones that came with iPad Pro. I'd paid 50$ more if it came with the 29W charger/cable instead.
Frankly, I may be done with Apple after this, they're really turning into the old Apple that I disliked. It isn't bad enough they're fragmenting their products into complex lineups, increasing prices on accessories and QA that's slipping worse every year, they're no longer any better than other companies for me.