On a rimfire the priming compound is in the rim of the case itself. For the firing pin to ignite it the brass must be thin enough that the pin can crush it. This severely limits the pressure a rim fire can hold. On a centerfire the primer is separate, in the middle of the base, and the flame travels up a small flash hole so the case can be much thicker and can handle much higher pressures. I don't know exactly what they load RF ammo to but I suspect it's under 10,000 PSI. Centerfire can go 60,000PSI+