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?
If you look at the growth graph of BTC in the last past year, then you will see that it is constantly an elevating one than any other virtual currency and the reason being is that BTC has the potential to provide you very huge profit than any other currency could provide. Moreover BTC transaction involves no third party seizure and provides you 100% confidentiality. BTC is an asset and millions are investing into it. These are all factors which made BTC very successful among all cryptocurrencies.