The disease, the progression of the disease, and any biomarkers of cancer are much better understood than CNS diseases.
Probably the most important is the ability to see the tumors and cancer cells via diagnostics such as x-rays, MRI etc. CNS has nothing like that degree of clarity into diagnosis and disease progression or regression. Some CNS diseases are getting better in genetic diagnoses but in regards to treatment there are still questionnaires and patient history logs compared to shrinking tumor size for cancer.
Case in point , how variable the placebo response was in the recent Rett trial. ADAS-COG is known to be highly suspect over the short term as are most observational scales.