Cost, volume, quality, and technology.
Molds for (industrial scale) pewter are usually made from vulcanized rubber, which is cheap(er) but experiences wear over time. The ones for injected plastic are usually steel, which is much more expensive but holds very fine detail for what is effectively “forever”. The plastic used for injection is much cheaper to buy and ship than the pewter. At high volume over a long time, steel/injected plastic is more cost-effective and better-detailed.
They’re also better for the consumer. Modern plastics and resins are tough and light enough that they can’t really hurt themselves with their own weight (unlike soft metal minis). The plastic Reaper uses also bonds better to paint than metal does, so a painted mini stays nicely-painted longer—the toughness of the material also plays into this. Resins vary on this, but almost universally do better than metal.
