"The more one suffers, the more, I believe, one has a sense of the comic. It is only by the deepest suffering that one acquires the authority in the art of the comic." - Søren Kierkegaard
In fairness, recreating Xen in the HL2 engine would require a lot of new models and textures, as well as coding for new enemies. Plus there is no real reason it would fit in with the rest of the universe created in Mission Improbable. I thought it was fine keeping the action on Combine occupied Earth.
I suppose it's a bit dull.