Actually, we know a great deal about reasons for failure, arguably more than the reasons for success. When you fail, you usually know exactly why, but if you succeed, then it was caused by a combination of many reasons, and you can only guess which of them were instrumental, which were detrimental, and which had nothing to do with it.
As for the reasons for failure, it is widely believed that the most common one is making a product that no one wants, and no matter how long and hard you try to market it, nothing happens. That's exactly what happened in my first big startup.
The second most common reason is that the team disintegrates - founders leave because of internal quarrels or some other conflicts, or to pursue other projects, or because they simply gave up. Happened to me so many times I actually believe this is the most common reason. It certainly is in the pre-seed stage, before the product goes to market, because the previous reason doesn't apply in such a case.
The third most common reasons for failure is when the investment runs out, and you can't find another investor. Paradoxically, this seems to happen more often if the investment was TOO LARGE, rather than too small, and came around too soon in the life of a startup. This is because if the investment came too soon, you didn't know what to do with it, and, if it was too large, it went to your head and resulted in a spending spree: renting luxury offices, hiring a bunch of unnecessary people (e.g. secretaries to sit on the phones for some reason), but most often trying to solve a much larger problem than you initially planned (simply because now you can). In many such cases, one day you suddenly realize that the money is about to run out, you're still not profitable, and the expenses are crippling. And you don't even have time to scale things down because you're over-committed. For example, you can't just fire people on the spot and be done with it (there are laws against it), you can't go back on existing contracts and lease agreements, and so on. Luckily or unluckily, this has never happened to me (and I don't intend it to happen in the future), but there are plenty of examples. A most famous recent example is the startup called "Better Place" that tried to introduce electric cars with replaceable batteries. They got a $1B (!) investment and used it all up building heavy infrastructure that completely covered three countries on three different continents (!). Took them 5 years before going to market, and the money ran out a few months after that, not because there weren't customers, but because the expenses were crippling and the revenues couldn't rise fast enough. They declared bankruptcy shortly afterward.
Those are the three main reasons.
Minor reasons include not being able to create the product because it turned out to be unfeasible, being trampled by competition, suffer crippling litigation or government crackdown, and more, but all these reasons are very rare. Certainly never happened to me.