Why do brain tumors form in glial cells instead of neurons?

Division: We have a finite number of neurons as an adult- they don't divide. Possibly that's why only dividing cells become tumors- glial cells. As divisions occur mutations occur and a cancer forms.

Both: In fact, there are tumors formed of neurons, and a significant proportion of brain tumors have cells which have both glial and neuronal characteristics. The conventional explanation for the higher frequency of glial tumors is that neurons in the brain are mostly post-mitotic, they don't divide, so they are less susceptible to dna damage causing cancer.