I think the main reason behind this is their popularity. Them being popular means that there will be a lot more people who will buy them because they are easy to buy and we all know the fact that if the demands of the bitcoins will rise the price of them will rise too because that how their volatility works. Then they grow much more that makes them a great investment and even those normal people who did not really invest try to biy bitcoins and so that makes the bitcoins rise.