I don't know the specifics for microscope lenses, I know a little about camera lenses, but I think it's largely high failure rate. It's a lot of mechanical grinding and heating and cooling with some final human checks. Each high quality lens has something like 10-15 lenses in it each which fails out pretty often. Also, shockingly, these lenses might still be hand assembled and checked? (I can't find a modern reference on this).

As to why it won't get cheaper anytime soon a couple reasons (at least what I'd guess):

1. Small batches and minimal incentives. It's feasible the high end microscope lens market just isn't worth disrupting. New processes could possibly make better lenses cheaper but just setting up the fairly massive tooling probably would eat up the margins. These lenses last forever so the yearly sale of these lenses is actually pretty small. (Funny enough there has been recent stories where competing manufacturers have started buying up some type of lens and actually completely drained ALL the available lenses of that type on the market, and that was a fairly cheap lens). Also people buying these lenses- labs and biotech tend to not care very much about price, so there is little incentive.

2. Replacement materials don't exist yet. The easiest way to beat the lens market would be to replace the glass lenses with plastic. This is what happened with phones/disposable cameras and is why camera prices plummeted. But at present this hasn't happened for higher end lenses.

3. There hasn't been cheap increased precision in machine movements really. So failure rates are unlikely to go down. If someone could adapt lithographic techniques to lens making it's possible it will get cheaper?

4. Kind of a dumb reason, but the biggest reason they aren't likely to get cheaper is that they haven't gotten cheaper. The process is about as automated as it can be and the microscope world is on the cutting edge of a lot of technologies (photolithogrpaphy, novel microscopy techniques, etc) if applying some technique like ML to some part of the process could make it cheaper, someone probably would have pushed it. There are also a lot of DIYers in this area. It's not a technological stagnant market. (I know this kind of competes with reason one, but what I'm saying is there is not 'easy win' to apply here.)

5. The big research in microscope/optics design isn't really focused on making high end lenses cheaper (As far as I've seen). It's more focused on stuff like making even higher end lenses that let you cheat the rules of optics, or like this paper, using low end lenses to do experiments that were impossible with more expensive lenses, making tiny/embedded systems, using beefy automated systems to image a ton of stuff simultaneously (can image a whole rat brain in like an hour now), and automating capture speed with the systems that exist (if your 100k system can image 10x faster and doesn't need a user it's like having a bunch of cheaper systems).

In summary, it's a well explored, small market, that no one is throwing money at and no obvious tech advances touch. Though this is largely speculation from talking to microscope reps and wandering around vendor fairs/automation conventions. Also I'm a biologist with an interest in optics not a microscope expert.