This question is a better fit for Molecular genetics = Molecular biology + Genetics
Why are mutant tumour-suppressoor genes and oncogenes generally recessive and dominant respectively?
This fact can be easily explained from the functions of these genes. Tumour-suppressing genes “watch” cellular proliferation, differentiation, genome integrity etc. So, they have to be active constantly. Every cell that is becoming tumour must inactivate main tumour-suppressor genes. And mutations that lead to inactivation are generally recessive. In contrary, cellular oncogenes (proto-oncogenes) are silent in the absence of the pattern of both internal and external stimuli. This means that cancerogenesis requires overexpression of proto-oncogenes. And activating mutations are generally dominant.