Because people may realize that trevon was not a little boy skipping home after buying some candy, but instead was 6'2, ~180+lbs, played football and was completely capable of assaulting zimmerman to the point that he may conclude he had to defend himself. We don't want those facts to get out.
Same reason zimmerman is a "white-hispanic", but our beloved president is not a "white-black".
Or it’s possible that he didn’t buy them.