Why do altcoins vary so along with bitcoins? Today bitcoin has dropped by 20%, so I looked for alternative cryptocoins with better performance. But, hey! Litecoin, Iota, Ripple, Ethereum, all of that has the roughly the same graph along time in USD, EUR or even BRL. See, for example (from https://www.coingecko.com/en):

Chart BTC/BRL:


Chart LTC/USD:


That does not make sense for me. "Oh, bitcoin is falling, I'll sell my litecoins" - I would buy altcoins.

Why altcoins (at least the ones listed above) are so related to bitcoin, instead of having independent variations?
The answer is that many altcoins are traded in BTC/ALTcoin pairs, not $$/ALT pairs. 

So if an altcoin is trading 0.1 BTC/ALT, and the dollar price of bitcoin falls, then the $ price of the ALT will fall an equal amount. 

Some altcoins trade against dollars as well, so their dollar price may be less coupled to the BTC dollar price. But for many of those alts, a large amount of the trading volume will still be from the BTC/ALT pair, and so the influence is still there.