If Glioblastoma Is The Most Common Brain Tumor Then Why

If Glioblastoma is the most common brain tumor then why do you still not know the cause and why no treatment that works? Being the most common you'd think they'd know more about it. My brother had his first headache on his 21st birthday August 28th, 2015 and then for three weeks thereafter until he lost his eyesight which got him to the hospital where an MRI found the tumor. He then had brain surgery the next day October 1st, 2015 to remove the tumor for a biopsy. October 15th he was diagnosed with Primary Glioblastoma Multiforme, grade IV. He died on Halloween October 31st , 2015. He didn't even fit any of the risk factors. 21 years old:(