Why is that?
The mis-used expression "mean-spirited" comes to mind. My understanding is that most of those who actually fought for the South, including Davis and Lee, eventually recognized the nobility of their opponents.
Nobody ever pinned any personl financial impropriety on either Lincoln or Grant. If they were evil men, it wasn't for personal gain. Lincoln actually gave his life for the cause he believed in. Grant risked his many times.
(Although Grant had many subordinates who were crooked during his Presidency, apparently he was personally honest throughout.)
Because most attacking us seek to do so from the alleged moral high ground.
"While most of those defending the Union position are perfectly willing to recognize that many Southerners, such as Lee and Davis, were noble individuals fighting for an inherently ignoble cause"
We do not believe it to be an inherently ignoble cause. I am not sure noble men would remain noble if they did that would they?