I'll try: The theory is that you can holster handguns. You don't have to sling a handgun; the holster is the most important safety (hence: good holsters are important, not gunbuckets, not serpas).
Also, they are smaller. If you fall, a handgun should not be hitting the dirt, but your body and you use the hand to keep the gun from being impacted. A rifle is going to take the fall. SMGs fall protection (to avoid open bolts loading themselves) is a similar issue
And the dirty secret that works for me and a few others: safeties on handguns are not reliable for this. They stick out, have little resistance, etc. so are easily knocked off, even in the holster. It is pretty easy to have the shooter's thumb disengage the safety by accident while moving as they have a firing grip all the time, so it's not a security measure as much as it would appear to be.
But also, yeah. A safety is a safety. I never really hold it against any agency that insists on a safety lever on their gun. IF it is used all the time. An M9 safety is dumb because it's too hard to use so no one wants to use it routinely, for example.