It is THE main and original coin, which is mainly known and discussed by the population, institutions and media. Satoshi Nakamoto's BTC blockchain technology is the revolutionary idea, which all other coins are based on. Of course, there are many coins, that have evolved and have a different purpose for everyone and there is a important place for them, too. Howevever, they are currently all dependent on the success of BTC. If BTC falls, every other coin suffers, too.
It is very logical that altcoins price depends on Bitcoin. But bitcoin network has too many problems. What about spam attack or BTC transaction fees? I love BTC very much, but now coin is too expencive and hyped...