Guess the fact that xen its self is a bag of worms which failed in teh original half life.
I can not imagine the amount of testing you guys have to do on it, seriously. You guys have to what, make sure the environment is right, fix the failed story, add bits of the story which while not in the original half life where still canon. possibly rework the ending, but knowing the canon you can’t do much about that.
Then there is bug testing.
Xen is an alien inviroment, look back at half life, look at all the strange and odd objects in it, then realize every inch of that has to be bug tested, ever event, every item. every thing. the low grave alone makes testing a pain as it adds more problems that need to be checked like making sure that long jumping into one of the spiked plants doesn’t launch you into space. (seriously that was a half life 1 bug, hard to do, but man flying from that was fun… to bad death always happened).
Not only that, but in xen there are many areas that the devs will have to either A reprogram AI specificly for, or B revamp completely.
example, in xen are the first 2 gargs that you have to kill with conventional weapons. Something that is impossible in black mesa. This means that the garg AI will have to be revamped for these parts, or a story element has to be added to give the player a way to damage them.
This is just one of many thing, there are also things like the passive vorts, the fact that an enemy can be an ally in section which means the team will have to find ways to convey that, the fact that at least one area has the manta’s firing off beams constantly at a rock, not only do they have to make sure the beam works there but they would have to find some explanation for it.
Then there is the locations them selves. IT is not like they can just copy paste the levels. the xen levels where generally either gravity based mazes on floating rocks or large cave systems, alone thoughs do not translate well into modern games. the fact you are having to swap from jump puzzle ot combat and even do both at the same time is something modern games dropped for a reason.
All of this is just a minor list, I can rant for hours on all the things the devs have to go through to get xen running.