Owning a Bitcoin offers no value to the holder other than the ability to buy something with it, but in almost every case imaginable there is a better alternative to Bitcoin out there. Usually it's much easier and safer to just use a credit card or a check or a bank transfer (or, heck, physical cash), but even in the case of illegal activities, you're probably better off using a different cryptocurrency that's less traceable.

The people who keep buying into BTC because they think it's fundamentally more useful than fiat currency are in a very small minority. Most of the people buying BTC are viewing it as a speculative investment, but the only reason they're buying it as an investment is because it keeps going up, and the only reason it keeps going up is because people keep investing in it. If it starts going down for some reason, even a little bit, the positive feedback loop could easily switch to a negative one and the bubble will pop. All the investors will start selling at once to try and reap their profits before it falls further.

Also, part of the reason the bubble has gone on for so long is that historically it's been difficult to short BTC. This may change with the advent of Bitcoin futures markets....