Well, even if the levels are well made and I've just got poor taste, Half Life was such a tightly designed package, introducing new weapons, things to play with (like the trains), enemies, environment modifiers at a steady pace.

Replacing a 5 minute level with a 20 minute level, even if it's better, ruins that pacing. There's just not enough content in the game to support it.

I agree Xen was by far the weakest of the original levels, but I don't think it's a coincidence that it was also pretty short. I think they knew it had novelty but no staying power and probably cut it to the bone.