it's because most alts have biggest liquidity on their pairs with BTC, meaning their dollar value is closely affected by btc's dollar value. That's why.
In certain countries, there is a restriction on fiat to cryptocurrency trade (and vice versa). Some of the users depend on stablecoins to avoid this complexity, but most of them restrict their trade to BTC and altcoins. Also, whenever someone converts his fiat to crypto (or crypto to fiat), it needs to be reported to the tax authorities (regulations and tax rules may vary from country to country). So a lot of people prefer to purchase altcoins with BTC. And finally, it is a big headache nowadays to deposit or withdraw fiat in the mainstream cryptocurrency exchanges (especially of you are living in a third world nation such as India).