I believe that they're popular mainly because well it was the first of all cryptocurrencies to be made. Honestly most of the other coins are scams and greedy ways that the developers make money. The only other coin I can think of that isn't a scam are dogecoins! What do you think?
How about etherium? have you seen the 1 year chart of it? Take a look and see that there is not much volatility in its growth(as compared to other altcoin).
Bitcoin have a very high price in market and its transitions are also very fast than any other currency if you want to earn from bitcoin so you can also earn a lot of money through different recourse and you can also learn a lot of things from it bitcoin is now available in many countries and its going to play a very good role in economy of many countries so join bitcoin to make yourself rich because its the only currency which can change your life.