It's a good question and the answer is pretty simple. Development takes a lot of time and most of that is spent making new content. At the same time sound is something that you can't just add in one place, it has to be the entire game. Even if we look past the coding side of implementing sound, the process of finding appropriate music and sounds is no small endeavor and if it's not done right it would detract from the experience rather than add. So at the end of the day it all comes down to wanting to spend dev time on the thing that matters most, as much quality content as possible each week.