my cross bike is a flip flop fixed/free. i usually use the fixed side for easy trails and normal riding, but i might try it in a race this year.
i use drop bars because i like having different places to put my hands and i like getting low and aero when i'm riding into the wind. can't speak for everyone.
if you're asking why (almost) no one runs flat/riser bars on their cross bike i'm not quite sure. some people do, and they work quite well for normal riding and racing. i think drops are just the default (plus they have the advantages i mentioned).