Price, availability, and no obvious advantage over brass ?
The only rectangular bar stock I can find on short notice is aluminum bronze 954. In the as-cast state, the properties are:
Yield strength = 32,000 psi vs. 45,000 psi for 360 brass
Machinability = 60% vs 100% for 360 brass
Thermal expansion = 9 ppm/F vs 11 ppm/F for 360 brass
Bronze can be heat treated to much higher strength but that prolly wouldn't be a good idea for bullet molds.
I have no idea how it would cast but prolly similar to brass.
There are many different bronze alloys so maybe there is something better out there, but 954 seems to be what is available.http://www.morganbronze.com/PDF_Files/954_spec.pdf