Different people have different preferences. Just like how some people prefer to use cash when dealing, some prefer debit card, some credit card and etc. Each method have their advantages and disadvantages, but people still use them. Same argument for bitcoins.
They still buying btc because they have money to invest and they believe that btc has potential to help them achieve thier goals. There are lots of reason why they invest and maybe we only have common reason. BTC been experienced crisis yet still able to recover.
It is the good need of bitcoin the bitcoin that keep the investors invest in it because we are now old using bitcoin so I have trust but as much bitcoin is famous new users are also not worry about their money they invest in bitcoin because they know bitcoin is the most promising and the most beneficial investment ever, I think we should make bitcoin famous and more and more so it will increase the price of bitcoin for us,