This is a combination of factors. There is always much karma behind human maladies. Karma of this kind is increasing rather than decreasing and this is because people continue to struggle so and do little when they are incarnated in the physical to work on their backlog of prior traumas. Most are ignorant about the reality of having lived other lifetimes and the robust, dynamic interchange of energies across timelines and how they are dragged down greatly by prior lifetime karmic energies projecting into their current life experience and challenging them to rebalance things. If they do not, their life experience will worsen and their future is guaranteed to do so as well.
There are increasing environmental insults as well contributing to the surge in incidents of brain cancer. This is largely a function of the bombardment by electromagnetic frequencies of those living in the modern era. With every increase in utilization, every added device, every additional router, microwave tower, WiFi hotspot, and so forth, the energies increase and take a cumulative toll on the body. This is sinister and intentional and the interdependence will not be easy to undo because of the great dependency everyone has come to rely on. No one will want to give up their technology for an idea even if the data look compelling. You have seen this play out before in the past where it took decades for changes in the law to discourage exposure to something highly toxic as was seen with the eventual anti-smoking campaigns and legislation. It will be no different this time around. There will need to be very, very compelling evidence from a huge number of victims in order for there to be a serious rethinking of the technology platforms and corrective action taken.