Everyone has his personal reason to invest into Bitcoin. To survive, hodl, become richer, because of believe into Bitcoin etc. But the key moment in this question is new and good opportunities that cash or banks can not give, because those two have borders, limitations and different nuances. Bitcoin give people new opportunities first of all - like time economy, bordeless transactions, income and so on.
Right, as there is no force on anyone to either invest in Bitcoin or not. those who want to invest in cryptocurrency, make sure that there is a huge risk in it and anytime the market will be crash or pump to the sky.