The reasons why are complex.
Whatever power is not explicitly granted in the Constitution to the Federal Government is the domain of the particular state (10th Amendment) and each state has done its own thing as it sees fit. The EMS Systems Act of 1973 does provide a standardised framework and some requirements that must be met to receive Federal funding but there is much variation as at the state level there is often additional legislation.
Each state has sort of adopted to what it needs for its purpose; for example when determining levels of certification e.g. Virginia used to have a "Mine EMT" or something of that nature, which would not be appropriate in say, Nebraska. At one point I petitioned IDHS in the Great Nation of Indiana to create an EMT-Tractor because I sick of going to blokes stuck under tractors who were still stuck when we got there because the local volunteer Firefighters were too busy closing the road in preparation for the Lifeline helicopter landing than digging ole' Jim Frank the farmer out from under said tractor, hmm looks clear, Fire have closed the road, yep look there, he's still under the bloody tractor I gather by the gaggle of people around it; Lifeline descending will call you again airborne .... OK I'm taking the piss but you get the idea
Historically the development of EMS in US was very much led by Physicians (originally Cardiologists like Nagel (Miami, Cohen and Cobb (Seattle) and Criley (Los Angeles)) and has just been "one of those things" that has just never died. Part of it is that EMTs and Paramedics do not have independent legal ability to supply or administer prescription medication to patients and need an instrument of delegation (a standing order). There is little Paramedic-led interest in changing things and there is also no single national body that represents EMS across the US; there are some pseudo organisations like NAEMT, NASEMSO, NAEMSE etc and this makes change, especially positive change very slow. The EMS Agenda for the Future: A Systems Approach
is a great example it was created off the back of something called the EMS blueprint (or near equivalent) in 1994 and nearly twenty years later there has been very little positive change. By international comparison places like New Zealand, Australia and Ireland have achieved 50x as much is as much or less time. This document did not really consider what is going out outside the US which would quite easily highlight the gross inadequacy of it.
It is the stated policy of the International Association of Firefighters and the International Association of Fire Chiefs to run EMS wherever possible. Both of these organisations are industrial unions. Let me repeat that, because it is quite important. It is the stated policy of the Firefighters union to run (including running by taking over) EMS wherever possible within the US. They have poured millions and millions and millions of dollars into this goal and pursue it aggressively; which I must say they have done a fantastic job of it as far as industrial representation and marketing go they are just expertly awesome at getting what they want, the problem is what they want is not good for patients or Paramedics. The Fire Service is an organisation steeped in tradition and structure going back many hundreds of years so be can quite change resistant. Also many Firefighters have no interest in EMS and go and get their Paramedic card to look good and get a Fire job; some places forces Paramedic certification on Firefighters (LA County, Houston, Dallas, most of Florida etc) so the Fire Service (and their unions) have a vested interest in keeping education standards as low as possible. The IAFC EMS Section is actually on record as opposing increases in EMS education, I did an awesome post about two years ago picking apart a document they submitted on the topic of EMS education but I cannot find it now, bugger!
In some places Paramedic education is as little as 12 weeks plus a couple hundred hours of "skills internship" (Houston) and in some places you must have a college two year degree (Oregon and Kansas). Most Paramedics out there get a quick watered down couple of weeks of A&P and a week of pharmacology. Because of this each Medical Director will give his or her Paramedics what he or she is comfortable with. I have had a medical director tell me that he does not know from where his Paramedics are coming e.g. a 12 week patch mill or a two year college program taught by a University School of Medicine so he has to take the view that the worst possible Paramedic in the world will be using the protocol he writes.
There is also little interest in Paramedic led research in the US which doesn't help as research drives practice especially with the push towards evidence based medicine.
Some states have state-wide protocols e.g. MA, GA, PA but they are not overall particularly good; they are at least a state wide standing order so that's a step in the right direction.