They don't want to spend the time, effort and $$$ to do that. Instead, their strategy is to make themselves look good by prosecuting a single, high visibility case.
Don't you know? Trading is for "entertainment purposes only". Your are not allowed to profit from this activity.
http://www.reuters.com/article/2015/05/20/us-flashcrash-trader-idUSKBN0O516120150520 Poor Nav now has no chance of getting out of jail before his trial....which may not be till NEXT year !! And Wow, that attorney for the USA is really intent on "getting him".