Because it's become a massive hot button issue now. 

But look at it this way...if an occupying army came into your home town and started erecting statues to THEIR leaders as objects of your subjugation and propaganda reminders of who beat you, you'd be pretty resentful, no?

Now imagine you're African-American and your city has all these statues of various Confederate traitors and slavery status quo upholders like Jeff Davis, Robert E. Lee, Nathan B. Forrest etc as DAILY reminders of how some people would like to see you...subservient and unfree. The whole "states rights" and "honorable Southern gentlemen" argument no longer holds water. It was about keeping another race of people down. 

That poor, lightly educated, bottom of the totem whites in the south that would have been spat upon by the likes of Lee, Davis, etc etc, have bought into this propaganda just makes it all the more head shaking.

Get it?