Slight tangent: a long time ago when I was into this hobby, Oshpark was usually the go-to instead of JLCPCB. What happened to them? Macrofab too if anyone's used them.
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.
Simple: jlcpcb can deliver you 10 boards for anything a hobbyist will likely need for 10$ including shipping, and it ships in 24 hours. Oshpark cannot compete with that.
OSHPark is bare board fab only but JLC does turnkey. I've enjoyed working with CircuitHub for turnkey prototypes. I'm not in love with the web interface (source files must be uploaded so if you have silkscreen that has dynamic parameters you need to make a ticket and the BOM parser misses stuff very often so every project needs an audit).
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.