I don't think anything necessarily happened to OSHPark so much as JLC became the default for a lot of people. I find the defining factor is just consistency mixed with capabilities. Even if one could conceivably get a specific PCB stackup quicker from a US company, being able to order basically any type of PCB (and increasingly more than just PCBs) from a single vendor is a pretty significant value-add.

There are US shops like Advanced Circuits that can turn boards around extremely quickly, but obviously you are paying for that speed. Most people ordering a handful of boards for a hobby project, or prototype are probably optimizing more for cost and consistency than shaving a few days off the lead time.

Once someone finds a vendor where they know the quality will meet whatever their own subjective minimum is, there's not much incentive to shop around. Being able to tack on assembly at JLC with a single checkbox also exponentially increases the desire to just stick with that vendor.