There is one Xen episode close to the end that is indeed way too big for its own good and quite boring, I will give you that, but otherwise I found the new Xen levels very well made and fleshed out. Lets be honest, the original Xen was quite lackluster...
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.