The people/departments that are closer to the money - or perceived as having a better understanding of financial issues - will generally have more decision making power in a capitalist structure.
Since marketing often involves things like MBAs, ROIs, NPVs, etc. while most designers and design departments do not traditionally have these as background or vocabulary, design remains subservient to marketing (and engineering, and finance…) - despite all the recent hype (see BusinessWeek, IIT, P&G, etc.) to the contrary.
(For example, look at the backgrounds of the CEOs for the Fortune 500 - you can find plenty with finance, engineering, or marketing degrees/experience - can you find a single one from a design background? “But we are making progress and reaching C-titles!” But does becoming Chief Creative Officer or Chief Design Officer put one in line for the CEO position the way that COO, CMO, or CFO might? No, because few designers can prove financial literacy or even a vague for-profit interest or EVEN HAVE ANY DESIRE to become a CEO - the one who has the most control and makes the important choices!)
Of course this has been recognized by some - hence the occasional conversation or post from designers interested in getting MBAs, or the Design Management Institute recently discussing design Return On Investment (but still mostly from a shallow, numberless perspective). May be a good start, but bear in mind that marketers have been upgrading themselves with MBAs and MSs for decades and both the marketing profession and marketing academia (and even advertising, to some degree) have been discussing and attempting to measure ROI for years and at least with Google/online ads, have found and demonstrated their holy grail.
Unless designers can “innovate” their way to better financial metrics, “design-think” themselves closer to the money, or “prototype” an accounting oriented vocabulary, it appears design has a long way to go and is playing catch-up to a moving target.
Finally, (to conclude my own rant rant grunt grunt!) if the all caps wasn’t clear enough, let me make the point again: if it is decision making power you want, then, taken to the extreme, you ultimately want the CEO position. Are you really interested in that level of ambition, of gaining diverse qualifications, of vicious competiveness, of accounting knowledge and responsibility, of leadership and general management duties??
Most designers I know are decidedly NOT, while more marketers I know ARE.