PEX is very tightly coupled to the X Window System. Not only was it designed in the context of X but its semantics depend on X notions of drawables, events, and execution requirements.
But X is not the only significant window system on the market. For this reason, OpenGL was designed to be window system independent. This means its API can also be used with Windows NT and future window systems. Application developers wishing to develop 3D applications for both X and Windows machines will appreciate having a consistent model for 3D across the two window systems.