Why is the Zcash to BTC value always relatively stable since its inception, but every other crypto I look up has its gold line (in coinmarketcap.com) wildly swinging all over the place? Also, the market cap is always way higher than the other two lines.
Zcash to BTC value is decreasing from its inception. At first 1 ZCash was 2000 BTC. It managed to get to 0.08 BTC a month ago. That could be the bottom, or not. No one knows for sure. It can go down more, since ZCash daily emission is still high.