If this thread is any indication, most investors don't understand nuances of cryptocurrencies, just Bitcoin and "not-bitcoin".

As projects like Ethereum continue to develop, I imagine it will diverge from Bitcoin more.

It should also be noted that Bitcoin has been highly correlated to the SP500 this year, despite being a completely different asset class.

I think it's because Bitcoin is kind of the 'de facto' cryptocoin. It's the easiest to exchange. Most exchanges exchange to/from bitcoin, not directly between altcoins.

They don't always. Bitcoin is the Sun in the crypto solar system - it provides the primary source of value. Many (maybe most) traders trade crypto like Eth against BTC (how many sats is Eth worth?) as opposed to USD or fiat. Check out a chart of Eth sats over time.