I'll give you a serious answer.
  It's a derivative of the requirement in the effort made by racists in justifying racism... that you have to dehumanize the people you're vilifying.  
  Food selection perhaps doesn't appear to be a natural component of that... or, you might expect that the foods selected would be ones that tended to have a much stronger racial identity ?  Down south, poor people eat a lot of stuff that northerners clearly don't understand... grits and greens, ham leavings, like chittlins en.wikipedia.org , but, those aren't identified as racial foods by racists seeking to offend, rather than as "soul food" or "southern food" ?  Why ?  Because, they're not things that are readily or obviously obtained by stealing them.  Particularly during hard times, things like chickens and watermelons... are things that are fairly commonly stolen... because they're fairly easy to steal. 
  authentichistory.com
  So the use is a backhanded reference to thievery, and with it all of the other racially defined elements of questionable morality that result from racists applying those sorts of labels to identify the entire race with the slur.  FWIW, if you grew up down south... you know for a fact that it isn't a fabrication that people steal watermelons.  You also know for a fact that the "sport" of stealing watermelons, as that frequently happens... isn't close to being a narrowly defined racial issue, while the blame for the thievery that occurs is likely to be far more narrowly defined than reality requires. 
  White people steal watermelons, too.  But, they don't tend to be the ones who get blamed.    
  The use of slurs in dehumanizing others is obviously still an issue... and, even if you're consciously unaware of it, there's an easily recognizable and pervasive social awareness of it.  Racism isn't what it was a generation ago, but, it also isn't close to being dead, and no longer an issue, as some might have us believe.  It is programmed into people... including those who think they're immune.  The link above shows the continued use of the concept up to the present day... with images of Obama.  You might deplore that depiction... but, you can't wipe the impact of it out of your mind once you've seen it.  The fact Obama got elected says more, in my opinion, about where we are in relation to race, today, than does the still easily demonstrable fact that we still have racists... both black and white.  
  The issues can still pop up in unexpected ways... and I have some personal experience with that.  
  I went to school down south.  The school had a "film" program that used student fees to fund showing free movies on the weekends.  The film companies sent the selected films along to the student run committee that selects and shows them... usually with a free trailer or two... an old cartoon, an old newsreel, and the students just show them without much thinking.   One of those free extra cartoon trailers they showed prior to a movie, once, happened to be this old Tex Avery cartoon from 1948: 
  classiccartoons.blogspot.com
  Watermelon scene just short of half way down... On the link, it is made much less offensive than it is in fact in the actual film, by virtue of the selection of the least offensive of the frames. 
  It had already been shown a couple of times before anyone said anything... 
  Protests ensued that lasted for a couple of weeks... almost turned into a riot at one point, before explanations could be offered and excuses made.   Accusations of racism, of course... even though the film committee was integrated, and the simple fact was no one on the committee knew or cared what the "freebie" cartoon from the film company was... until after it had been shown.  
   
      
    
   
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