I'm sure the main reason MSK picked paxalisib for this study is...

I'm sure the main reason MSK picked paxalisib for this study is because, unlike alpelisib, paxalisib can actually cross the BBB and can therefore act on brain cancers. Alpelisib can affect head and neck cancers (those outside the brain), but cannot affect cancers within the central nervous system, so alpelisib is useless for brain cancer. If paxalisib can have any effect on brain cancer in this study, it will still be better than alpelisib for this purpose. Of course, the more effective, the better, but there really aren't any standard treatment options yet that I'm aware of.