Many cells associated with a variety of cancers have mutations in the p53 gene. Why are mutated tumor suppressor genes, like p53, considered recessive mutations

it is a recessive mutation because one functional copy of the p53 gene is sufficient to maintain the normal phenotype (i.e., one functional copy of the p53 gene can protect from cancer)

Explanation:

The p53 gene encodes a protein that plays a critical role in controlling the cell cycle. It is for that reason that the p53 gene is referred to as a "tumor suppressor" gene which acts to suppress tumor cells. Moreover, it is well known that when individuals inherit only one functional copy of this gene (instead of two normal alleles), they are normal but also prone to develop different types of cancers during early adulthood. The Li-Fraumeni syndrome is a genetic (inherited) predisposition to cancer due to the inheritance of a mutated p53 gene copy.