Demand mostly. But there is likely going to be a correction soon. At least, that's what SHOULD happen.. but you never know these days..
I agree that demand is the main factor why it is keep on increasing. As of now the price of bitcoin is keep on increasing and I think it will continue on increasing. I think if more investors will invest the price of bitcoin will rapidly increase and may lead to high income. The supply and demand of bitcoin is the main reasons why the price was totally expensive.