Competition. Because all startups need a frontend to get users, there are many independent firms hiring in that market, and because demand is high, there are many developers as well. Because the market is both liquid and very competitive, devs need to SCREAM VERY LOUDLY to be heard (minus a few folks who get snapped up early by highly desirable employers like Google, Facebook, AirBnB, or Uber and then quietly sit and work, often moving more toward backend as their careers progress).

There are relatively few hardware-level companies and hardware-level software engineers. There are relatively few FPGA companies and FPGA engineers. There are relatively few OSes, and relatively few OS-level software engineers. Within these fields, everyone knows who the good developers are based on their work.