I have commented some of those reasons before. They mostly have to do with two things, the impracticality of having the IEMs made in the USA and living in Europe, which makes any fitting issues, repairs, etc an expensive and time consuming affair, and hygienic reasons. I've seen at work all long term complications that hearing aids (using the same type of in ear molds the customs have) produce. Supposing I'm careful and I clean the molds frequently, and never fit the phones if the ear canals are wet or suffering the slightest condition, still those hard devices, despite comfy, stimulate wax production and act in the same way as Qtips do, they push the wax way into the canal, close to the drum. Though I have many otologists at work who could clean my ears with good care, I still prefer not to need that.
Soft tips and a more external fit, would prevent those issues, hence my preference for universals albeit I know they won't isolate the same, and possibly won't give the same performance.
Then there's the curiosity for knowing how good they are...
Anyway, I don't think JH will make them in an universal format, since at his site he seems to be stick to the benefits of custom fit. We'll see