It's at least partially a design and engineering problem. Sig shoehorned the hammer-fired P250 fire control unit into the P320, which is striker fired. The P250, being hammer fired, uses a fully cocked hammer capable of setting off a primer when dropped, and the P320 (to my understanding) also uses a fully cocked striker, meaning less trigger input is required for firing.

Hammer fired guns are capable of doing this safely because they have a sear geometry that requires moving the hammer back against spring pressure with trigger input a very short distance before the hammer drops. Along with a functioning sear block in case the hammer slips off the sear without trigger input, this makes them very safe.

Basically every other striker fired gun on the market uses a semi-cocked striker with a trigger widget and sear block, which is a copy of Glock's design, and it's quite safe.

Sig deviated from this design without fully proving it out. Their guns don't have trigger widgets, which allows the trigger to move under momentum when dropped, causing repeatable firings. The fully cocked striker design leads to a shorter, crisper pull, but a sear slip leads to uncommanded firings, unlike a semi-cocked design, which doesn't have enough energy to fire a primer.

Combine this with poor control of manufacturing, intermingling of parts designed and intended for different calibers, as well as factories in the US and India with varying levels of quality control and poor spec for parts to begin with (metal injection molding for fire control parts), and safety critical systems like the sear block have been shown to not be 100% reliable. It's a system of cascading failures resulting in a firearm that's unsafe to carry loaded.

There were no standard trials for the M18 so Sig didn't even need magical sample guns.

For the XM17 program that ended up adopting the P320, the military skipped their normal production verification testing.

See: https://www.military.com/daily-news/2017/07/05/glock-says-be...