After some thinking and reading, and since the construction of a cowbell relies heavily on welding, I believe that the answer to my OP question lies in the easiness to weld steel by TIG welding, MIG welding, or even spot welding. All these techniques are well suited to industrial production (of any scale) and produce a mechanically strong and durable weld.
On the other hand brass or bronze "welding" is more "soldiering" than welding. In fact "brazing" brass might be a better name than welding brass, and in general requires a flux (joint filler).
From my reading, it appears that such a jointed brass or bronze would produce a "weld" area somewhat brittle/fragile, and have a far lower mechanical strength than a steel weld, especially considering the percussive assault resulting from using a beater on the cowbell.
So, basically, even though one could envision a brass/bronze cowbell with superior acoustical/musical properties as compared to steel, the realities of cowbell/campana manufacturing and playing make brass/bronze cowbells an unrealistic and short-lived proposition.
Please let me know of alternate explanations as well as objections.