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?
because the network effect must not be underestimated. Actually usefulness of bitcoin currency is growing with the square of participants inside it (Metcalfe's effect). For this same reason most money flows into Bitcoin and not alt-coins, which gives Bitcoin even infrastructure advantage (new applications, etc....).