Because Bitcoin is the first, accepted by many users and stores. No altcoin comes close at this point.
Yes because bitcoin is the first altcoin ever made and it is trend because it is well establish coin than any other and it is on top and have higher price among them, but for saying it will be the future currency well it not gonna happen physical money is still the best because mostly of all people prefer the money they can touch and bitcoin is not secure for now because theirs many treat for its existance and strength Bitcoin is unstable for now and anything badthings might happen to it.