Tungsten, as an alloy component, is a very small percentage of steel and not significantly affecting the weight of steel shot.
Steel shot, when compared to lead, is low density, which is why high velocity is important for pellet energy.
Steel is a poor compromise for density brought about by the economics of shot production.
Truly heavy Tungsten alloys have always been very expensive, and quickly fade from the marketplace. There have been many versions over time. The best was "Nice shot" which was soft like lead but high density.
New generation Bismuth shot seems to be a better product than the original version, but we have limited and unreliable supply in Canada of factory Bismuth shells at any kind of reasonable price.
None of which answers the original question - I would guess that it is hard to produce relatively soft cast iron spherical pellets of consistent size.
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