Both.

The US never had a comprehensive EMS system as it was never seen as an essential service, both because EMS is expensive to run (especially in the healthcare/insurance/taxation environment the US has) and because there was significant lobbying against it (there is money in EMS on a large scale if you operate it in a very cut-throat way).

But the recent downturn in healthcare availability and county-tax-income in rural regions and the dwindling volunteer numbers and enshitification of medicine have all done their part in making the whole situation so much worse.

There is actually a good study showing "ambulance deserts". (Just as a reminder: That does not mean that no Advanced life support provider comes..it means that no Ambulance is available at all. So not even one staffed by an EMT-B and an emergency medical responder. And we're not talking about "what happens if we need two ambulances at the same time)