Why is the Nobel Prize not awarded to mathematicians?

Six Nobel Prizes are awarded each year, one in each of the following categories: literature, physics, chemistry, peace, economics, and physiology & medicine. Notably absent from this list is an award for Mathematics. The reason for this conspicuous omission has been subject of extensive speculations. A source for this is http://www.snopes.com/science/nobel.asp.

The most convincing explanation is the fact that the Swedish King had already established a prize for mathematics. It may also be that Nobel was more interested in areas of science that in his view had direct practical application.

The prize in mathematics considered the equivalent of the Nobel Prize is the Fields Medal, though it is not quite comparable as it is awarded to younger researchers, under 40.

See http://en.wikipedia.org/wiki/Fields_Medal.