Snicko is not advanced enough thats why they don't use it. Firstly, they can't compile the information fast enough, it usually takes a while. Secondly, sound moves slower than light, so that's why sometimes the noises will appear after the ball has passed the bat. When England reviewed one of the nicks (not Uz's, Clarke or Smith's), it was a good example of this. There was a definite noise, but no hotspot. The noise came after the ball passed the bat and it seemed to have come from nowhere.
When compiling the information, they need multiple camera angles, because as you saw on the Usman Khawaja dismissal, a one-dimensional view gives a different perspective.
I remember a few years ago, I think Pakistan were playing England, there was really good example of this. From the front on view, it looked like he had knocked the cover off it. From a side on view, you could clearly see the noise came about a metre passed the bat.
Snicko would have been useful for Khawaja's dismissal. You could see a more solid block of lines when the noise occurred. An edge usually shows a huge outlier for a line.