First bitcoin is the one of the oldest cryptocurrency that exist until now, it really matters because it already gain the trust of the user and investors. Secondly it is now a very flexible cryptocurrency, in fact you can use bitcoin in many ways most probably as a medium of payment and always compares to physical money.
Being first does play a major part in order to gain trust but we should also considered the usefullness of it. however bitcoin is indeed useful in so many ways, that is why bitcoin is still in demand thesedays. not only bitcoins but also other coins like altcoins especially etherium, is also indemand today because etherium is indeed a pretty useful coin in terms of investment , trading and even payment gateway.