Every manufacture is using some type of Brass or Copper Alloy, there are a couple reasons. Aluminum would be good due to high thermal conductivity and light weight, but has a low melting point, wont work. Titanium is light weight but, has a low thermal conductivity and is expensive.
Brass/Copper Alloy has a high thermal conductivity, wont melt like Aluminum, costs less than Titanium, low corrosion for application, higher thermal conductivity than titanium..so it's only down fall is the weight.
My question, in not being a materials engineer is, which type of brass, i am a Mechanical Engineer(Tool and Die)and a CNC machinist, we usually use 360 for most things, but that has lead in it, I'm sure no one wants lead in there burner getting red hot, possibly gassing off etc.
I would use stainless or Titanium but the burner bell needs to reach a temperature that upon liquid fuel entering the bottom would heat and vaporize the fuel to a gas. Some mfg. add a separate copper tube to pass through the flames, i want to avoid this, so the high thermal conductivity has to be there.