Peoples are holding BTC because they are waiting it to grow up and make some money, also they can invest in mining and get double.
It is mainly because they have some amount of Bitcoin to hold for, I mean you need to have a large number of it. Having huge amount of it means that you are now a major holder of Bitcoin. It could also affect the price of it, if you could just analyze it, these major holders greatly affect the price of Bitcoin. If they suddenly convert or sell in mass number of it, the price significantly change.