Because in a few years the popularity of bitcoin will become much higher. And if this happens, then we will see a price increase, directly proportional to the growth in demand for the Cryptocurrency.
Looking at the present growth in bitcoin we can easily assume that the future of bitcoin will be even brighter and it will be holding a higher price too and if truly happens then we would regret in future for not investing in bitcoins at the right time and this is that best time to invest in bitcoins.