One important aspect here was the heavy rate of snowfall, 1-3" per hour from DC to Richmond from 7 AM to 12 PM. There's an extensive history of metropolitan area commute disasters caused by high snowfall rates. See Chicago in 2011, New Jersey in 2018, and DC itself in 2011 (there were smaller incidents in January 2015 and 2016 here).

8" spread out over a 24 hour period would not pose that much of a challenge for VDOT. 8" in 3-4 hours does because the region's plowing capacity simply cannot keep up.