No salt or gravel + drivers who are unused to the conditions + everyone got let out of work/school at the same time = total gridlock.
Took me three and a half hours to get home (five and a half miles). And I count myself one of the lucky ones!
You know, I grew up in Colorado, where there is often snow on the roads.
Now I live in a town that rarely gets snow. A few months ago we had the biggest snowfall in 40 years - about six inches.
The roads here were worse than I ever saw them in Colorado - this town has no trucks equipped to spread salt or sand (and stockpiles neither), and has one snowplow. The plow is paid for out of federal money for the airport and cannot be used off the airport until the airport is fully cleared.
Needless to say, our predicament pretty closely mirrored yours, albeit on a much, much smaller scale. I-15 was closed for about 24 hours, complete with lots of cars, trucks, and school buses spending the night stuck in the worst portion outside town.
Some places are well equipped to handle this sort of thing, and people are equipped as well with the right kinds of tires. People who live in those areas sometimes can't seem to understand what a difference the equipment makes. It is not that people in snow states are tough - it is that their cities and highway departments have spend a boatload of money preparing for it.
Its not the severity of the weather, it is how unusual this weather is (well, that added to Atlanta's usual mess of traffic).
No doubt there will now be calls for the city of Atlanta and the state of Georgia to buy snow and ice removal equipment, or to always shut down at the prediction of poor weather. If they do that, then in 20 years new politicians can blame the old (current) ones for wasting money on equipment that might be used once every decade or so, or for shutting down schools and offices when weather predictions are bad - especially when weather predictions turn out wrong.