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.
It really doesn't other than that in the simple minds of investors all cryptos are essentially the same (i.e. tied to Bitcoin) regardless of their respective business ecosystem, technical attributes, usage, etc.
Basically, my small stash of XLM nearly trippled in value in the last weeks. The same keybase handhout has floated between 50$ and 230$ in the last year or so. Most of the price changes have very little to do with the amount of progress/change in the Stellar ecosystem; which actually is not that bad given that some non scammy people are trying to use it for real stuff and they've made some nice progress moving the platform and OSS forward. Actual merit has done little to nothing for the value of XLM. But somebody coughs in the Bitcoin ecosystem and it suddenly jumps by a few hundred percent.
Pretty much any time there's a change in value for XLM it is because something is going on with Bitcoin. Eth2 is a bit more widely used but it seems to have similar issues differentiating itself from Bitcoin.
The ecosystem is basically being driven by so-called crypto experts circle jerking bitcoin to new highs and lows via extremely biased web sites passing themselves off as independent blogs/news channels that are pretty much crooks for hire willing to write whatever in exchange for crypto. Most of that BS is so detached from reality at this point that it's hard to see any patterns other than some whales moving bitcoin around to influence the value of bitcoin and orchestrating lots of hype around this so they get to dump after their pump at exactly the right time.
I mean, it really shouldn't make sense, because Bitcoin is a completely separate currency from ETH... but it's still way too deeply connected to 'anything crypto' in people's minds, it seems.
Not really anymore. If for example you look at the ETH markets, the top 12 trading pairs by volume are with national currencies, and the first market with ETH/BTC is down at the 13th spot.