The full size SD card slot is important to photo/videographers, a market that Apple seems to be taking for granted recently. The range of "professional" users that the Macbook Pro is actually appealing to seems to be getting narrower and narrower over time.
Can attest as a non-pro photographer (but as someone who cares a lot about photography), that the SD card built in is not that important. I tend to use a dock/reader anyway because most of my cameras use CF or mini SD. True, full sized SD cards are actually not used in any equipment I have now.
CF has always been used in high end bodies, but most of them still have a secondary SD card slot. And these days SD is fast enough to be used as the primary card and there are flagships like the Sony A9 that only take dual SD cards.
