CroMo fatigues to rapidly, it's not even that great for bicycle frames, good ol' steel works the best for solid rear axle. There's a lot of weight cantilevered off the end bearings, I don't know of any other racing type group aside from quads that don't run wheel bearings on the rear, we of course have axle bearings but they are general closer to the center of the axle than to the center of the rear tires. I myself run a 1" steel axle out back, I know some that run 1 1/4" axles (again steel) solid or hollow . Finally, aluminum is not a good axle material either because of the moment arm that is our axle.