because alt coin prices are based off of BTC price. Most of them will have -btc or /btc next to its code. This means price is based of btc. when btc does a bull run some alt coins can still rise in value but most wont be able to. but when btc drops alts value increases relatively to the drop if their price is based on btc. Furthermore high volume coins such as btc, eth and ltc are traded based on usd. so btc/usd or eth/usd. Unlike altcoin/btc.