It is the pioneer in cryptocurrency so the buyers think that bitcoin is safer that those new cryptos. It is the best investment for those traders because of high growth rate. And no one owns bitcoin unlike those ICOs that are potential scam.
Bitcoin looks like the leader of the crypto currencies and is the current leading coin. Bitcoin is better than any other coin because it has the most room for growth and it is what most of the people know. People invest most in bitcoin and maybe invest in altcoins to gain more bitcoin because out of all cryptos, bitcoin has the most highest price.