The writing has been on the wall wrt SD cards for a while. They are used by an ever smaller segment of the user population and will eventually go the way of zip drives.....And for that reason, Apple did not find it beneficial to splurge on and include such a seldom used hardware component.
I don't believe that's the reason SD wasn't included. Note the following:
1) Dell includes an SD card slot on the XPS15 (their competitor to the MBP). Dell is a big company, and has the resources for, and knows how to do, market research. Its PC (desktops/laptops/notebooks) market share, by units shipped, is more than twice Apple's (by dollars it would be less dominant, since Apple devices are more expensive). Dell wouldn't have included an SD card slot (taking up precious space on their machine, which is about the same size as an MBP) if there weren't a reasonable demand for it.
2) Apple also doesn't include USB-A. Regardless of what you want to argue about SD, clearly there is still a significant demand for USB-A.
Taken together, we can conclude that Apple doesn't base its port decisions on what specific ports people most commonly use. [Indeed, by excluding USB-A, it's deliberately ignoring what most people use.]
Rather, it bases its port decisions on its broad-based design philosophy. And it's design philosophy is two-fold: (1) We're going to look to the future to determine what we believe will (or should be!) be the upcoming standard. (2) Using that standard, we're going to give you four powerful, flexible ports. With the right adapters, you can connect whatever you want to the machine. I.e., we're not going to give you the ports you think you want, we're going to give you the ports we think you should want.*
There are pluses and minuses to this approach:
It makes the machine very flexible, and gives it significant potential connectivity, but at the same time reduces its convenience for those employing common devices that use the current standards: You have to keep track of which of the current common standards you might be using, and make sure you carry the right adapter along with the machine. E.g., you can't just plug in a USB-A key.
It also makes the device more clean and beautiful by itself, but at the same time makes it uglier and less compact in actual use. The ultimate example of this was the "trashcan" Mac Pro. By itself it was very compact, neat, and clean. But in actual use, because of its limited port selection, you couldn't do the clean thing and plug directly into the machine, you needed to add one or more outboard port docks. They have, fortunately, rectified this on the new Mac Pro.
[*This is consistent with Apple's historical design philosophy, which is to give the customers what Apple likes, and assume that, once customers use it for themselves, they will realized it's the better way, even if they didn't see it at the time. Sometimes this has worked brilliantly (and Apple wouldn't be the company it is today without this), and sometimes it fails utterly -- as it did with the butterfly keyboard, and as it did when Apple unwisely tried to Apply their "we know what you want better than you" philosophy to the pro market, resulting in the trashcan Mac Pro. With they new 16", they've held to this design philosophy when it came to the ports, but acceded to customer demand in many other areas, particularly the keyboard, smaller bezels, physical esc key, better thermals, and maxed-out battery size.]
