It's simply because a significant percentage of the trading pairs with good volumes are still the BTC/altcoin and ETH/altcoin trading pairs. Though we've been seeing growing trading volumes with stablecoins(USDT and USDC) since the past months, despite Tether(USDT) still being quite "controversial".
Additionally, all altcoins technology is based on bitcoin 's technology
When bitcoin adoption grow, altcoins potential adoption grows as well. As bitcoin is more tested and developed altcoins also get benefited by that.
On the other hand, as governments increase regulations on bitcoin , all the potential negative effects are also applied to altocins.
That is what correlation is about.