Because the only people who care about 40 yard times are fans, or those who are in the football community. Since football timing is not FAT, sports reporters who are used to talking about football 40 would not view Bolt’s FAT time as being as impressive as it should be. Since his FAT time is pretty similar to what the top football recruits are accomplishing with football timing, it would discredit the Athletics World: “That Bolt guy can only run 4.24?? Olympians aren’t any faster than the NFL Combine stars!”