Brazillian Rosewood and African Ebony have traditionally been the two woods of choice for most guitar fretboards. The primary reason is their hardness on the Janka scale, making them much less susceptible to wear. Due to them both being protected species (in one form or another) some manufacturers are gradually moving to a product called "Richlite", such as Gibson / Epiphone. Although generally well accepted for electric guitars, acoustic guitars have always been made of wood, and having a synthetic fretboard just doesn't seem right.
There are other very hard woods which are more readily available (Lignum Vitae, for example) but because they're lighter in color, they haven't been widely used or accepted. Here's an interesting article on Brazillian Rosewood, as well as one on the declining status of dark woods in general:
http://www.wood-database.com/lumber-ide … -rosewood/
http://www.wood-database.com/wood-artic … ark-woods/
Several have mentioned how short-term commercial interests have typically trumped longer-term thinking. If not too bright, human beings are at least consistent!
