What's stopping people from using bitcoin, you ask? The majority of people don't know what is bitcoin and they might as well never heard of it. People tend to fear the unknown, because the aren't informed about bitcoin, they fear that it's a scam or smth. Without proper knowledge about bitcoin, you'll never understand its advantages over fiat money. And because the majority of people are too lazy to study bitcoin, then they just ignore it.
Some people think is scam? I would say they must be wrong. Bitcoin is not a scam, I also thought the same when I first heard about it from my friend. The way he explained it made me think its scam, but its not. If its a scam, many users would have discovered that since. But people's imagination is real stopper I too believe.