My first question is: Where are you seeing so much M4? I feel like I could name all the regular production models using M4 without running out of fingers to count on.
Anyway, adding to the discussion:
Toughness depends on how high the hardness is set from the heat treatment - most production 20CV / M390 / 204P blades aren't going to be run at such a high hardness that you're going to run into chipping issues from general use.
Tool steels like M4 are well-suited for situations that would be abusive or highly damaging to typical, generally stainless steels. If you are looking for a knife that will handle terrible situations* well, go for M4. If you're looking for a general EDC knife, the M390 family of steels is an excellent choice - if not superior, when one considers corrosion and sharpening issues that often come with tool steels.
Armchair sociology time: I'd bet the reason you see M4 as often as you do* is that people want it, and companies like to make things that people want to buy. Competition cutters like tool steel, and, as much as we might not like to think so, people are influenced by the choices of those who excel in their field of interest.
* Not actually very common, honestly