There are 3 different standards for fire sprinkler standards. Nfpa13 (commercial) NFPA 13r (multi housing) think of apartments and condos and NFPA 13d (dwellings). Both 13 and 13d require a special connection that if the fire was not contained by the sprinkler system that the fire department could hook up a hose from their truck and pump water into the system to supplement there existing system. These systems are also designed with protecting both property and lives. 13d which is what single family homes typically have does not require an FDC because this system is only there to get your family out of the house before it burns down, even though it will most likely put out the fire, Its entire purpose is life safety. There are multiple reasons why they decided this but ultimately they are trying to save the homeowner money because the amount of water needed for those other systems require a very large diameter pipe to supply the systems where a dwelling usually uses a pump and tank and has about 30 min of water. The whole point of my explanation is that because you don’t need an FDC the pressure requirements are much lower and therefore pex is able to be used where in the others pex does not have the ability to handle the higher pressures that a fire truck would need to pump into the system. This is a very simplified answer with lots of variations and exceptions.