Size, speed, features. You can have a small FPGA for about 2$. You will however be limited in the complexity of the design you can implement because it has limited gates and pins. And limited speed. Like normal processors if you want faster, bigger and more features the cost shoots up. Combine that with the limited market for big FPGA's and they can be very high in price. Mind you that many also have things like a multi core ARM processor on board since many designs need a processor to control the circuit. Same with many interfaces. Would be a shame to use the limited gates in the FPGA for standard things like that.