Why did people buy Tulips? What was the reason for the demand? They bought on pure speculation and hoping for price appreciation. There was no Utility to the Tulips. That is definitely a mania.
Why do people buy bitcoins? Sure some speculate but as discussed in the original post people buy Bitcoins to transact. It is a currency, probably the best currency available to us. That demand for it is created by its utility. So, will the price of Bitcoins in dollars fluctuate. Sure, but as long as the demand is there it's function as a currency will keep it's value in a steady ascending trend.