Who says that ETH and BTC are digital coin Ambassador, maybe itu would be appropriate when you say that ETH and BTC is more popular than the other coins. Actually, the other coins are also in running because to make ETH and also BTC blockchain are still balance, you can Imagine if we only have ETH and BTC, and then all of people in the world are doing transaction by using both of them, You can imagine how high the fee of btc blockchain and also gas prices for ETH, and how long it takes to transfer them to another wallet. The existance of the other coins will make us to have an alternative coin when we wanna do the transaction with a low cost.