Because it's least susceptible to reporting bias, and being an extreme, also offers a base point for interpolation, to independently estimate numbers of lessor crimes, especially violent ones.

That's true, but if you don't focus on murders, 2020 was one of the lowest violent crime years on record looking at SF, CA as a whole, or the entire US, so 2021 having an increase in crime is nothing more than reversion to the mean.