Why is stationary armature preffered you commercial generators than moving armature?

The problem with a generator having a rotating armature is that the armature must be connected to its external load via slip rings and brushes. As power station alternators generate voltages up to 25 kV or so, and supply hundreds of amperes, the resulting arcing at the slip rings would be severe, and would require short maintenance cycles. Accordingly, it makes more sense for the armature to be stationary, and connected directly to its load, and have the field winding rotate instead, because the voltage/current applied to the field winding is quite low.