The geographic location of both battlefields made it easier to preserve them in that condition.
Yes, they are more remote than Manassas. Manassas is now in the middle of a large populated area that wants to expand.
Fredericksburg, however, is a small town and I think that Battlefield should have been preserved. But the area of the field that has been overbuilt with houses, is the hill where thousands of Yankee Soldiers were slaughtered. Could be that no one wanted to remember that. I can't say that I blame them.
With all the crap that's going on in New Orleans, I hate to see any Civil War sites, artifacts, statues or whatever destroyed.
It's history, nobody today sees it as racism, except those seeking political advantage or favor.