@akflightmedic hit the nail on the head.
I would add that a large factor that gets surprisingly little mention on these threads is reimbursement: EMS agencies just don't make a lot of money.
Each EMS agency bills the patient and/or their insurance for each transport. Some of those bills get paid at the rate they were billed for, many of them get settled for less than the amount billed, and many do not get paid at all. So each gency, depending on how good of a job they do getting reimbursed, ends with an "average" payment that they receive from their calls. This is the amount of revenue that they receive divided by the number of calls they had to run in order to get that revenue, and it is be used to plan budgets......and set salaries. Anyway, that average revenue per call is a surprisingly low for many agencies. So they end up trying to make up the difference with volume or with IFT's, which tend to be much more secure streams of income.
Bottom line is that EMS is an expensive service to offer, with lots of overhead. Revenue, though, is often low. There simply isn't a ton of money to give out in salaries.
Salaries are higher, of course, where the labor market is tilts in the favor of the employees (i.e., there are fewer potential employees relative to the number of positions) - or where the agencies make more money.
But until the system of reimbursement gets overhauled, there will never be much money in EMS.