People buy it because they expect the price to go up. The price goes up so more people buy it expecting further gains. The fundamentals arguably don't justify such a high price, and so you have a speculative bubble. Eventually, the bubble will pop and most investors will lose money.
I suspect BTC is a bubble because everything it does can be done better by other things. I also suspect that Lightning is fundamentally insecure. If you analyse the true intentions of the people who use it, there's no sober purpose; they just want it because they expect the price to go up. It's not about helping the developing world; because if it was, then there are better but more boring ways of doing it. Hence, it's a bubble built from speculation.
