Because controlling timing of signals is easier than controlling level. You can take advantage of the fast and low-jitter clocks already made for digital electronics.
If you have a multi-bit DAC, it's difficult to made it linear. With a 1-bit DAC, it's much easier, because you only have two possible output values. And if you run it fast enough, you can apply a simple analog low-pass filter to the output and convert speed to bit-depth.
If you have a multi-bit DAC, it's difficult to made it linear. With a 1-bit DAC, it's much easier, because you only have two possible output values. And if you run it fast enough, you can apply a simple analog low-pass filter to the output and convert speed to bit-depth.