I have this problem but in a different sense.
So many digital devices have some sort of pen/touch functionality that carrying a sketchbook is more of a novelty than a necessity. My main work device is a Microsoft Surface Pro 2, and before that, several tablet laptops that allowed me to untether from a desk and work wherever I want. When you have a mobile workstation, that you can carry in the same space as a notebook, and can fulfill all the same functions of that same notebook (and more), why do you need the notebook?
Even on a portable level, the idea of using the Nintendo DS and its successors as a sketchbook was what allowed me to convince my parents to get me one back in highschool (even if pictochat was the best there was at the time) Nowadays, if I want to doodle on the go, I just pull out my 3DS, boot up Colors and go from there!
That's not to say that there aren't advantages to the traditional approach; for one notebooks don't have battery limits or solar interference from glare, but when most of my time as an adult is spent indoors with ready access to a plug, those disadvantages are negligible.