This is the probably the most mysterious line in the entire poem.
The ditch is nearer to whom? Shaw? The speaker? Us, the readers?
He's not likely talking about the literal ditch, because we're still in New England and Shaw was killed and buried in the South.
The ditch must stand for something then. Maybe it's the ugliness and lack of humanity of the soldiers that threw Shaw and his men there. Looks like that kind of sentiment is coming closer. There are small signs that we've already read in the poem that hint to this idea: the digging (like ditches) for construction in the middle of Boston, and the mention of the underworld.
There are no statues for the last war here; on Boylston Street, a commercial photograph shows Hiroshima boiling
Woah. When we read line 54, we don't really know where in time we are. Lowell seems to have jumped forward past the Civil War, and is talking about a more recent ("the last") war.
We find out by reading on that it's World War II. How do we know this? Well, the United States dropped an atomic bomb on Hiroshima, Japan in WWII, one of the most devastating moments in US military history.
There are no statues commemorating the heroes of this war, though. Why? Maybe the speaker thinks this war is less heroic? Maybe he thinks the world has changed, and people no longer honor war veterans. It's hard to say.
What we do know is that, instead of a statue reminding us of the war, we see a picture in an advertisement on a street in Boston (Boylston) of Hiroshima after the bomb was dropped. Lowell uses the word "commercial" to describe the picture, meaning that it's intended to attract customers, make money.
Making money from a photo of a devastating event seems pretty icky, and pretty much the opposite of erecting a monument in its memory.