Hey Lauren –
That is a good question. First and foremost this is not once size fits all, this is more so my personal beliefs based off of the progress I see in others as well as myself.
The thing is for those who train under me we already train low volume so to speak and the progress is there within the current approach so why change it / if we pulled the volume back further to be able to increase frequency I think we would be getting our sessions short in terms of truly impacting that muscle group? That is my reasoning its working and working well. Another thing and this is just personal for me I really struggle mentally with going into the gym knowing I have to train 3 bodyparts in one day. Not to say its not beneficial, and its not a mental weakness thing on my end but I have always struggled connecting with second and third bodyparts on a given day.
The bottom line to all this to me is that we need to be getting stronger and we need to be recovering. If you are recovering and you are getting stronger you are going to grow. What I see often times is that people are recovering to produce the same amount of work over and over and not recovering to be able to improve that level or quality of work (in terms of loads / volume ect)