We can't. As SE networks have expanded, we've invariably partitioned up the topics perhaps to a fault and we're now stuck with overlap.
I think branding is as much UX as it is Graphic Design, for instance.
And good code certainly plays into UX, so wouldn't say that's completely off topic either.
But I think we have to draw lines somewhere, even if they are fuzzy and somewhat arbitrary.
It looks like there was a marketing SE site proposed: http://area51.stackexchange.com/proposals/51786/marketing
Currently it's in closed status, though.
However, there's now a digital marketing proposal: http://area51.stackexchange.com/proposals/65811/digital-marketing
Maybe that question fits there best?
(Crazy Idea that might be terrible: Perhaps this site should be 'UX and Digital Marketing'. It'd be nice to see those two disciplines come together since they often need to work hand-in-hand to succeed.)