It's probablya combination of reasons really... after all, can you think of anything in society/technology that happens for a single reason??
Reason #1
Social Taboo. The Jedi were such a persuasive and highly regarded influence on the galaxy on a whole that there was undoubtedly a lot of common mythology regarding the order and their tools... including lightsabers. Superstitions don't disspear as a society becomes more advanced, they just change to fit the times.
Reason #2
Difficulty of use. A lightsaber in the hands of a untrained person is a menace to everyone and everything around him/her. To use one with true skill you almost have to be force senstive. As for melee... yes, no-one's gonna block your swing... but if a gammorean using a vibro axe takes an accurate swing at you... you can't actually block that swing either. If you try you just slice through it and the peice you chopped off hits you anyway. For a non-jedi, a force pike is a better option... doubly so when you consider the fact that lightsabers ARE as rare as they are.
Reason #3
Laws. Lightsabers, due to their lethality and undoubtedly due to the influence of the Jedi and the Mythos surrounding them, are probably heavily regulated, quite possibly outlawed completely in many places.
Reason #4
Difficulty of manufacture. While they can undoubtedly be manufactured by machine, such machines are probably expensive as hell, heavily regulated, and generally hard to get a hold of for various reasons. And it takes a Jedi to build one by hand.
There are probably dozens of other reasons, many of them tied to the ones I've already listed. Just keep in mind that ina society as complex, diverse, advanced, and as OLD as SW, reasons for most things are going to be multifaceted.
Anyway, just my 10 cents.