because bitcoin currently sucks as a currency (unstable prices, difficult to get, exchange rate fees, lack of vendors)
the only reason to use bitcoin is:
a) a speculative investment (betting that the value of bitcoin will increase)
b) buy drugs.
One might by tempted to list money laundering, but I don't think bitcoin is good for that at all.
The difficulty in obtaining bitcoins makes using it to launder unpractical.
Which means, most people are interested in bitcoin because they think it will increase in value, which in turn means these people think bitcoin will be useful as currency one day in the future, even if it is not now.