Bitcoin has it's place as the first digital currency, but it is old and with a lot of weaknesses in relation to newer coins... fess are very high, speed is not good and privacy is almost gone... besides, the only use that it has is transactions...there are a lot of projects with other uses besides money...
it is the largest because it is the better known and first and the one with most hype...
if crypto is to evolve and go global, bitcoin will have to loose some of it's importance