Yeah, that was the lazy answer. The more correct is, because grass contains stuff (mainly lignin, reminds me of this i found this morning) that's hard to digest with a normal stomach. So cows & co. have multiple gastric ... sections(? Mägen), the first of which contains microrganisms specialized in breaking down lignin. The following gastrics are more the usual bio-chemical kind, to digest the microorganisms.

In short, eating grass needs a specialized process and you can't eat anything else as main food source, (they do occasionally eat a chicken or critter).