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.
Not all the time, honestly I'm surprised that you have said they are always relative in terms of their price. I think you just looked to it some other day and that's what you have noticed.
But not all the time they are always relative, sometimes they are not and shows inverse relation.