It has nothing to do with courage, it has to do with the Real World. They didn't attend because they are politicians. As such, their job is to attend to the politics of here and now.
It is a simple, straightforward calculation: which is more useful? Pander to the folk who see anything dealing with the Confederacy as evil and racist, or pander to the folk who think Lincoln is Satan and Sherman is still burning plantations in Georgia, or don't go near the issue with a ten-foot pole. They could denounce the Hunley as a symbol of a racist State, praise the Hunley sailors as heroic warriors, or just STFU.
If they have any political ambitions at a national level, they will choose the third option. Rule One for politicians is don't commit on wedge issues. To expect them to do so, without visible political gain, is folly unless their base is made up of one of the two previous camps.