Ok, there are two foreskin diseases. The first is phimosis, which means your foreskin is too small. It hurts. The second is balanoposthitis, which hits diabetics. It's basically gangrene of your foreskin and is as horrific as it sounds. Both of these are basically genetic.

My suspicion is that certain strains of Middle Eastern populations tended diabetic. This tendency, like religion in that time, was basically genetic. I suspect someone noticed that balanoposthitis was traumatic (worms-in-dick-level body horror). The "obvious" solution was to get rid of the foreskin. Call it "heath", "hygiene", "covenant with God" but some families probably started it as preventative medicine. Better get clipped too early to remember than too late to ever want to reproduce. Or something like that.

I don't think the "reason" remains foremost in people's minds now. It's just a "rule" (which is always a bad sign, imo).