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.