Let it not surprise you that most of the population that is into crypto currency, which is speculated to be a large number of people, might just know about bitcoin, but a lot of them don't have any single volume of bitcoin in their crypto holdings; it's all filled with altcoins. 
 
The demand for Altcoin is limited depending on what the person going into crypto currency has in mind in the first place. If it's just for them to chase a large profit, then Bitcoin might not be an option for them, but if it's for privacy and something they want to hold for a long time, they will definitely have some Bitcoins in their portfolio.