Because males possess only a single copy of the X chromosome (genetically, males are XY) whereas females possess two copies of the X chromosome (genetically XX).
As an aside, in case your teacher drops this on you, males receive their X chromosome from their mothers, since the Y chromosome would have to come from their fathers.
Lastly, a female would be considered a "carrier" if only one of her two X chromosomes was associated with an X-linked disorder (one normal X chromosome and one that was associated with the X-linked disorder). Carriers generally are asyptomatic, but pass the abnormal X chromosome to their male children in roughly 50% of cases (since there is a 50-50 chance of their passing the abnormal X chromosome)
Hope this helps!