What is the evolutionary explanation of the fact ostriches have two toes (and only one fully developed) and simple feathers while emu and cassowaries have three toes and two-branched feathers?

I refer you to the recent answers to many of your previous questions where I have repeatedly made the point that there is never going to be one "evolutionary explanation" for any phenotype. Evolution selects for traits to allow that animal to best occupy a particular ecological/environmental niche.

In this case if you look at:
http://www.earthlife.net/birds/struthioniformes.html

it give some possible explanations to your questions.