According to me the unstablity in the market value is the main reason due to which many people are not investing in bitcoin.Sudden drop in price and moreover no surety that when will the market value increase adversely affect the number of investors of bitcoin. Moreover their is no control on the transactions as their is not central body to control it which also create a sort if fear in the mind of investors .
Maybe their reason is that they do not trust the bitcoin. They can not seen the advantages and other perks of using bitcoin as a medium where you can invest in. As of this moment many people did not realize the potentials the bitcoin are up to. Soon bitcoin will be on top and can possibly beat even the currency we are using as of this moment.