Better science. Many older manuals had pressure readings taken from the CUP method, which *can* be way off if little things are even a tiny bit off, like the hardness of the copper cup or a little misalignment in mounting it in the fixture.
Litigation?? Remember, the guy's *don't* get sued until someone blows up a gun. Sure, the first line of defense is that the guy's scale was off and he over-charged the round, BUT....what if it were you?
We learn more, and quit publishing stupid loads once we find out they're stupid.
IIRC, one powder maker found out that their test-bed .357 Magnum was a single specimen that generated safe pressures (in the days of technically-calibrated "pressure sign" readings) using loads that were close to PROOF level in min-chamber production guns. They backed off their Max charge listings immediately!