i think it's mostly 2 points (both of which have been touched on)

1) most people here use their fixed gears for city riding. the issue is not that steel is better than alum for fixed gears, it's that steel is better than alum for city riding. good steel is a much more forgiving ride over bumps and curbs and potholes. while alum is stiffer, most people are not TRULY mashing their way through town like they would be in a cat 2 race or on the velodrome, and prefer the give of steel over the lack of give in alum.

2) i think a lot of people just prefer the look of simple round tubing, which isn't really an option on alum fixed gear frames.