As MobileCrunch notes, even if this is true, there could be a few reasons for this, from a bad economy to just a bad seasonal trend (that could pick back up as soon as a few months from now). But more likely, it seems, is that Apple just hasn't sold iAds well to advertisers. iAds, as Steve Jobs told us when they were first announced, are a premium product -- they are interactive, well-designed, and of course, the cost is premium as well. But advertisers may not be entirely sold on the power of mobile advertising quite yet, so Apple is likely having trouble finding advertisers both willing to take the risk and big enough to afford it.
The silver lining on this so far is that despite having trouble selling the ads, everything we've heard so far is that the ads do pay off. Not only do they increase brand engagement, but they reach solid customers in exactly the right ways. Still, if Apple can't get advertisers to sign on the dotted line in the first place, it won't have any of those success stories to talk about.