So right now SD would be the most common format in cameras. Yes there are increasing numbers of high end cameras which use CFExpress, but they are the minority of bodies in peoples hands. As important as true 'pros' are for buying the high end MBP, I imagine Apple envisages selling a lot more to consumers, simply because there are a lot more of them, and most of them will be carrying around cameras which use SD cards.
So the option was to have no card support (when it is a heavily requested feature), include a CFExpress port (which needs more bandwidth and most purchasers won't have a camera currently that use it) or use an SD slot (which the majority of purchasers are using cameras which use SD).
In several years time as CFExpress becomes more ubiquitous that may well change, but by then Apple are hoping you will also be changing your MacBook Pro for whatever they are offering then.
To be clear, as I stated earlier in this thread, adding SD does nothing for me and I think it's kind of retrograde to add support for a dying format, but I don't much care. I won't use that feature. It appears to have precluded a feature I could have used (another USB port). The new machines weren't designed for me alone. I'll deal with it.
That said, there's been a ton of people complaining about the lack of SD slots for the past few years and arguing that the MBP isn't "pro" because it doesn't include that slot. In that context, I'm simply voicing and supporting my view.
The return of SD kind of surprises me. This hasn't been Apple's approach in the past-- they've tried to motivate the industry to move to newer and better standards. The regression to SD is counter to that history.
I do believe that there is some circular reasoning driving this. I think people like SD cards in their cameras because their computers make it easy to use them, and people like SD slots in their computers because their cameras have one. If Apple put in a CFExpress slot, which is clearly where we're moving to in the future, I think it would help drive convergence to the better standard.
And I think it's fine to argue that there are more non-pros with SD cards than pros using other things, and that's a fine reason for Apple to support it, but that argument can't easily coexist with arguments about whether the MBP is "sufficiently pro" or arguments that "the industry" wants SD. "The industry" is leaving SD. The more Pro the camera equipment, the more quickly you'll see the transition to better standards.