That might mean that PG (and the other YC folks) would have to move as well. AFAICT, YC wants to be close to the founders for physical meetings. It might not be easy for YC folks to move (they have their lives, etc.).

Also, Boston is a niche: it is home to some bright tech people. YC is close to them, while the valley is away, so YC is the obvious choice for Boston students. That gives YC a competitive advantage at least in the area and makes it distinct. Moving to the valley would make YC more generic and less of a brand.