Because this is engineering, and in the real world you have to weigh the benefits.
There are always 4 criteria in the balance here: speed, capacity, size and cost.
SD, while an awesome format, is physically very thin. This makes the capacity and speed criteria hard to hit without causing cost to go nuts.
CF had the advantage of a large size. This meant you could up the capacity and speed without affecting cost to much.
The problem is CF is physically too big. Pro cameras have multiple memory slots, supporting 2 CF cards takes up ALOT of room, room that is much better used in other ways.
XQD is slighlyt more then the length and width of an SD card, it's just thicker. This, combined with a physical interface capable of PCIE speeds makes it possible to get a card that has high capacity, awesome speed, for not to crazy a price. You can't do that with today's tech in the physical size of an SD card. Give it a few years, it'll come.
FWIW, every single device I use is SD based, in the consumer world it IS the defacto standard. The format is greast because it has 2 options, normal SD for bigger items, and micro SD for smaller items. The beauty is the microSD format is electically compatible, so just use a passive adapter and you only need one card reader to handle all your devices.
This of course assumes you don't buy a product that doesn't use SD, I don't, it's one of the reasons I don't even look at Sony products.