Simply because of the network effect. That means bitcoin has value because people want it. If people reject bitcoin and go to the 'next big thing' then not only the price will collapse but and all the market in a short period too. So what miners do with all of these investments in machines and infrastructures? How do they pay their loans?  I found and share with you a well documented article which entitled as The 7 Network Effects of Bitcoin. To my point of view it is worth to take a look.