Why are tumor suppressor genes recessive?
I was typing out an explanation, but then realized that this text book does a much better job:
Lodish et al. 2000. Section 24.2: Proto-Oncogenes and Tumor-Suppressor Genes in Molecular Cell Biology 4ed. WH Freeman
Proto-Oncogene:

Conversion, or activation, of a proto-oncogene into an oncogene generally involves a gain-of-function mutation. At least three mechanisms can produce oncogenes from the corresponding proto-oncogenes.

Point mutations in a proto-oncogene that result in a constitutively acting protein product
Localized reduplication (gene amplification) of a DNA segment that includes a proto-oncogene, leading to overexpression of the encoded protein
Chromosomal translocation that brings a growth-regulatory gene under the control of a different promoter and that causes inappropriate expression of the gene

An oncogene formed by the first mechanism encodes an oncoprotein that differs slightly from the normal protein encoded by the corresponding proto-oncogene. In contrast, the latter two mechanisms generate oncogenes whose protein products are identical with the normal proteins; their oncogenic effect is due to their being expressed at higher-than-normal levels or in cells where they normally are not expressed. However they arise, the gain-of-function mutations that convert proto-oncogenes to oncogenes act dominantly; that is, mutation in only one of the two alleles is sufficient for induction of cancer.

Tumour Supressor Gene:

Tumor-suppressor genes generally encode proteins that in one way or another inhibit cell proliferation.... Since generally one copy of a tumor-suppressor gene suffices to control cell proliferation, both alleles of a tumor-suppressor gene must be lost or inactivated in order to promote tumor development. Thus oncogenic loss-of-function mutations in tumor-suppressor genes act recessively.