You know what they say: "Don't buy the first version of something, wait for a revision/update".
While between geeks like us we have to remind ourselves about it, the reality is that most consumers also have figured out the same thing by themselves. The second, improved version of something is usually the most popular.
There's a lot of people currently interested in the current iPhone 5 that will see the 5S as a bonus and a signal that the time has come for them to take a dive and buy an iPhone.
Apple also invests a lot of time and money in new manufacturing methods for the iPhone back casing, and they usually have problems keeping up with the initial demand because of low yields in the first model using this new process (like the iPhone 4).
It's not that they're bad at it, it's just that Ive seems to like to push the envelope in term of manufacturing precision using metals and unconventional techniques. Most other manufacturers just change the plastic mold for the back casing when they want to make a big generational change.
Because the iPhone 5 casing is now much easier for Apple to produce, they'll be able to launch the 5S in many more countries while still meeting demand, just like the 4S.