Bitcoin is the original cryptocurrency. That alone gives it an advantage over other cryptocurrencies. Even if there are many other cryptocurrencies which have started following the footsteps of bitcoin, none of them are as successful as bitcoin was when it was released in 2009. The reason for this is the fact that Bitcoin is safe and secure and people trust it enough to use it just like they use dollars, euros or pounds.
I think what gives Bitcoin the advantage over other crypto currencies is that it's the first introduced blockchain, so it took the first step towards blockchain technology adoption. Thus, it has the respect of all crypto holders whether they like it or prefer another crypto currency. Another reason, in my opinion, is the anonymity of its creator, which is considered a good point for legal reasons.
I personally like Bitcoin and prefer it over the others for now, but still think that one day another crypto currency will replace it, maybe Ehtereum, which is developing very well, maybe another new one. However, Bitcoin will always be remembered as the first viable blockchain.
Being the first one to experience big success is the prime reason. The success took place after years, but to that time it lasted. Even before bitcoin several attempts in different forms have been innovated but nothing seems to be efficient as well as worked well. As said, the creator being anonymous is really a good thing for the success of bitcoin. With time technologies will change based on the requirements of the people. I'm not sure of how bitcoin will be overtaken by ethereum. However it is matter of time as we were to experience higher demand for cryptocurrency as people are getting more awareness about it.