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?
Being the first cryptocurrency, Bitcoin received a lot of media attention that pretty much all other altcoins don't get. Because of this, a lot more people invested into Bitcoin compared to altcoins and raised the price of it. This just kept the ball rolling as more and more media attention was gained when the price of Bitcoin was rising. That gave a lot of people a little push that they needed to invest into Bitcoin and this then made the price even higher. I would say that Bitcoin is the most popular currency because it is the most valuable cryptocurrency and it has received the most media attention.
To answer your second question, most other coins are not scams, but I can understand why you might think that. Many of the other coins have a lower value which makes people think that it is a crappy investment because it will probably just crash at any moment. But what they don't realize is that it was the same when Bitcoin first came out. Most altcoins die out after a short period of time, with some exceptions of course. And they don't really gain that much attention, other than ethereum, which makes them unknown to 99.9% of the population. The combination of a low price and a lack of attention mean that altcoins are a lot less popular than Bitcoin.