Why are Calico cats only females?
The color gene is attached to the X chromosome.
Each cat gets an X chromosome from its mother and either a Y or an X from the father. Since the gene for fur coloration is attached to the X, male cats, having only one X chromosome, can only be one color. Having two X chromosomes, females can express both color genes, making them calicos.
Technically, a cat can end up a male with all three chromosomes and thus can be a male calico, but this is incredibly rare.