1) They are either of very low quality compared to others at the top, or are complete garbage and are generally not worth buying at all
2) They are generic me-too rip-offs of existing popular brands or products
3) They try to fill a niche that doesn't readily need to be filled or isn't well-understood
4) Lazy marketing/communication or, worse, "build it and they will come" complacent genius attitude
There is a reason that companies like taptaptap can continue making _at least_ a half-million dollars in revenue on their apps consistently. You've got to take a look at the mid-section of the app store rankings to get a feel for what the real, unhyped apps are doing, and why they are successful.
What I mean is, take a look at the apps at #75 to #150 on the app store chart. You get a good cross-section of apps in all categories _and_ price points. Let's also remember that, if figures from past blog entries about app store sales-to-rank correlations generally hold true from day to day, apps in that mid-section are generally getting between 400 to 1000 sales a day. Link that to a sales price of, say, 4.99, and you're looking at some serious money per diem, even after Apple's cut.
That's where the true beauty of the "Gross sales" chart comes into use. It's less of a popularity contest or crapshoot, because I've seen indie companies with little to almost no brand relevance clinging to that mid-section rankings for well over a year, and some of them are priced above 99 cents (in a few instances, well above).
What it really boils down to is quality. If you don't have anything worth selling, I guess no one will buy it. Furthermore, it's really hard to determine if what you're doing is worth buying to the general iPhone or Android smartphone user. Put another way, if you take everyone playing in your space, would _you_ choose your work over the competition? If not, why not? Because I can tell you this right now: Most, if not all, of the apps that have ever kissed the top 10 rankings on the app store were made by 2-4 person teams and took a few months of full-time work to complete. The ones that stick around update _constantly_ or are established brands. There are some flash in the pans that make it to the top but if you look closer, they are either building hype elsewhere (such as with a free facebook or flash app) or fills a niche in Utilities that heretofore haven't been well-served.