Earlier quoted context omitted.
There are WAY more trucks/SUVs out there than 3-child households. And I'm not sure what you mean by "wagons don't meet emissions requirements" - wagons would meet the same regulations as sedans.
Wagons are heavier and less aerodynamic than sedans. When coupled with the same engine they are less fuel efficient (compare the E450 sedan to wagon). The fact that they are heavier and sell poorly means they will only usually be available with the large(r,st) engine (no E350 wagon in the US). Note that I used the Mercedes because it's the only non-compact wagon I'm aware of for sale in the US.
* Wunibald Kamm has entered the chat.