There's a lot of things which makes bitcoin the most dominant crptocurrency. First, and the most obvious, is its undeniably high value (and continuous rise of its value). I don't think that anyone won't like to invest their money on bitcoin with its volatile value.
Second, bitcoin is more preferable its popularity. There's no doubt that bitcoin is the most popular crpytocurrency ever invented. When people hear the word "CRYPTOCURRENCY", the first thing that will come into their minds is "BITCOIN". Bitcoin is also the main reason why there's a lot of new altcoins appearing today where most of them want to overtake bitcoin in the top list of cryptocurrencies in all criteria (which I think is improbable to happen). Also,bitcoin is dubbed as the Father of All Cryptocurrencies.
Lastly, the usability of bitcoins. As time passes by, the number of establishments, agencies, and countries that accepts bitcoin is increasing. This is because bitcoin is easier to use and understand compared to other altcoins. As long as you have a btc wallet and you know the features of your wallet, then there's no hustle in doing your transactions.