It is because of lack of knowledge and information that they probably didn't know. Yes for us there's a big potential to earn in bitcoin while others they still living with a classic way. It means other people have no interest in bitcoin because for them working in their full-time jobs is a good way of getting money to live.  There is a huge potential in bitcoin or cryptocurrency as a whole but if you are already into it and invest some hundreds during its peak price and saw how it's price fall maybe you will hesitate to say the opportunity here.
But I think if we share our knowledge and experiences in cryptocurrency maybe all of them become interested and trying to find their luck in the crypto world.