Bitcoin was the pioneer coin that was adopted with high success rates. before all these other altcoins came into being. it paved way for the development of other cryptocurrencies to develop. Having proved that the Block chain technology was reliable,BTC basically introduced and demonstrated to the world that decentralization from the regulating authorities is possible
The fact that it is the very first cryptocurrency, it can be considered as the mother of all cryptos. It is most popular because all other altcoins are based on bitcoin. All its price are based on bitcoin. So if bitcoin does or fall, so as other digital currency.