(1) Bitcoin is the prime mover - so obviously, it won the hearts of early adopters and it slowly gain some attention, reaches ATH in 2017 that no other coin has gone. So it has perks of being the first coin in the market.

(2) There were attempts in the past, ETH (Flippening) XRP (Rippening) and BCH (Bitcoin Cash), they didn't succeed because they don't have the support of crypto community although they are already that big during their attempt to overtake bitcoin

(3) Altcoin price are based on bitcoin's movement, or all altcoin has been traded with bitcoin as their pair.

(4) Most altcoins are shitcoins anyways, that's why that stigma sticks to the altcoin market.