TP53 is a recessive gene and is not located on the X chromosome, why would people who inherit just one mutant copy of a recessive tumor-suppressor gene be at higher risk of developing cancer than those without the recessive gene?

Since it is a recessive gene, tumor suppressor genes required the inactivation of both alleles, this is usually accomplished by the mutation of one allele and a targeted homozygous deletion of the second allele. This leads to an effective inactivation of the suppressor genes with the mutations acting a a dominant negatives. This way, patients are put at a higher risk of developing cancer