Because ginger hair hasn’t had a chance to spread through the population, and it probably never will. Red hair is due to a mutation in a gene which produces the pigment melanin. Some mutations in this gene have become more popular in other animals, for example the pocket mice, for which a change in hair colour can help them camoflage themselves in certain terrains. In humans, however, it has spread through a significant amount of the population, because if doesn’t give any disadvantage. It hasn’t taken over the population either, because it doesn’t offer a reproductive advantage either.
When this MC1R is changed through inheritence of 2 recessive genes on a particular chromosome (number 16, if you’re asking!), the pigment called ‘pheomelanin’ will be in greater quantity in the hair, which gives it a reddish, gingery colour.
To answer your question of why not many people have it:
It is a fair bet to say that the fair-skin associated with red-headedness will generally be found in mild climates in the North, where sunlight is scarce. Hence, in very sunny regions, like Africa, the genes which result in redheadedness (and pale skin) are not selected for, because of the high skin damage that it would entail:
So it will not tend to spread throughout sunny regions of the world – which does rule out a fair part of if (although people who have the gene and migrate to those parts of the world are helping to mix it up with the native population!). Add to this that the gene itself is rare, and you can see why not many people have red hair! It’s important to distinguish, though, between a red hair being rare because not many people have the gene, and it being rare because it is recessive. Recessive just tells you what gene gets expressed in the person – not whether it is there or not. So reasonably recently news stories about the gene for redheadedness dying out are incorrect, because they are based on the assumption that ‘recessive’ means they are likely to disappear.
However, enough people have the gene for redheadedness to allow us to state that redheads are here to stay.