Currently you can't buy alts without bc. So that's why their prices are so linked.
Once there more ways to buy alts with fiat, you'll be seeing less of a price dependency
Actually there are loads of alts you can buy with fiat (dollars or euros, or korean won). Ethereum, litecoin, bitcoincash, monero and others can all be bought directly.
More platforms now, allows us to buy them in some physical stores or just through their websites, but we are used in the common way that btc is the main way to bought them, but eth here can be bought in a convenience store easily like btc did.