Reconciliation. The US wanted to make sure the war was “history.” They wanted no hard feelings. Remember the southern states weren’t merely defeated, they were utterly and totally crushed, their cities were leveled, their economy destroyed to such an extent that, at best, they didn’t recover until the late 20th century. The civil war still impacts many area, such as why no southern university is “Ivy League” (their endowments were wiped out by the war), to the fact that post-1865 northern English is the American standard and souther dialect is considered inferior and a sign of poor intelligence.
Loyalists, such as Benedict Arnold weren’t a problem since they left and moved to Canada (though the US was always on the brink of war with Canada for the next century, and only became buddies in the early 20th century). It took a century for the US and Britain to become friends again.
In Britain there was no reconciliation after their civil wars of the 17th and 18th centuries (no statues honoring Jacobites, and Cromwell remained controversial until 250 years over his death) – and the aftereffects of those civil wars are still a major factor in British politics.
The US didn’t want the losers going around with a chip on their shoulder, and honoring the memory of the fallen rebels was a major part of reconciliation.