I'm not exactly sure of why coins are not produced with pure zinc, but here's my speculations on the matter:
1. In pure form, Zinc is a hard metal, but also very brittle. Beating a pure zinc coin with a hammer would probably result in a shattered coin. I would imagine that by forming an alloy the metal would be much more stable and durable.
2. Compared with other metals, the melt value of Zinc is fairly cheap. Coins made with pure zinc would likely deflate in value due to their inexpensive composition. You can call a quarter a quarter, but if has the same metal value as a penny, is it really a quarter any longer?
Alright, so that's basically the logic that makes sense to me. Not sure if it's the exact answer you're looking for, but hey, it might just be a start.