I just adopted a female orange tabby and I knew going into the adoption that they are very rare. Why is it so special for female cats to be the orange tabby colour?
The color orange has to be present on all X-chromosomes that the cat has. A male cat has 1 X-chromosome, while a female cat has 2 X-chromosomes. Therefore, a male cat has to inherit the color orange only from his mother (his father's color does not matter). A female cat must inherit the orange color from both her mother and father, and so that is statistically more rare (but not exceptionally so).
It's explained in more detail, with approximate percentages for the UK, in David Greene's book Light and Dark: An Exploration in Science, Nature, Art and Technology.
The presence of orange fur arose from a mutation of the non-orange gene on the X-chromosome into the orange gene. The effect of the orange gene is to replace dark colours with orange and to suppress the action of any non-agouti gene. Consequently, the orange colour is never uniformly dark but is always associated with some kind of pattern, although the contrast between the two shades of orange may be slight. Because male cats have only one X-chromosome, the occurrence of the orange gene in a large population equals the faction of male cats that have orange fur. This is commonly around 30% in Britain. On female cats the equivalent orange coat occurs only when the non-orange gene is absent, which requires both X-chromosomes to carry the orange gene. The fraction of such females is 30% of 30% which equals 9%.... Female cats with no orange gene on either X-chromosome form 70% of 70%, i.e. 49% of the female population. The remaining 42% of the female population have the orange gene on one X-chromosome and the non-orange gene on the other. Instead of one gene being dominant and the other recessive, both genes contribute to the colour. Patches of orange fur and of dark fur occur side by side, and the cat is described as a tortoiseshell or 'tortie'. The combination of these two colours, with our without additional white patches, is found on female cats only. The very rare male tortie has two X chromosomes and one Y, and is invariably sterile.
Another book, John Bradshaw's Cat Sense: How the New Feline Science Can Make You a Better Friend to Your Pet, is also in agreement.
Also note from this logic, for a cat to inherit the color orange from its father, the father must be orange. However, if the cat inherits the color orange from its mother, the mother can be orange OR tortoiseshell.
This answer is an addenda to Zaralynda's answer. The reason why also depends on the special way the X chromosome is handled in the body. The other chromosomes are always both 'in effect' in the body. Since the male only has one X, the body has developed so that only one X is 'on' in any particular cell. The other is turned off, called X-inactivation.
This doesn't happen on the first cell division, but later, and that cell and all its children will use one X. The neighbor might well choose the other X, as the choice is random. So a female with a mixed gene set has patches of orange fur where the orange gene was selected and patches where the non-orange gene was selected. This produces the tortie and calico color pattern. There is a similar genetic disorder in humans on the X, which affects sweating, so that women with this have dry patches and sweaty patches..