Writing 90 degrees is equivalent to telling the motor not to move...
If you try writing something else in your loop, the ESC will probably go into either an error or a calibration mode. As others have said, you need to let it arm. This requires a constant neutral signal for half a second or so (maybe a bit more depending on your ESC). What is defined as neutral depends on how the ESC has been calibrated, but is probably close to a 1500 microsecond pulse. Try adding writeMicroseconds(1500) and a delay in the setup routine. Then write whatever you like later.
If this doesn't work, make sure that the ESC is calibrated correctly. It needs to know the max, minimum, and neutral signals. The manual will tell you how to do this.