I think it is because bitcoin has proved in the past few years that it should be trusted that is why it gained so much popularity. The popularity also affects its price because many investors are investing on it. Other new altcoins are not reliable because some of it are useless or sometimes called as shit coins. Not all new releases must be invested to,you need to understand it first before investing. That is the exact opposite of bitcoin, you do not need to worry about investment because it is legit and trusted.