Why Can't Some Birds Fly?

Birds like chickens, penguins and ostriches can't fly, but it turns they were once able to. How did they lose this ability?

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Evolution Made Ridiculous Flightless Birds Over and Over (Discover Magazine)
"The ratites are a group of birds that includes the ostrich and emu, as well as the kiwi, rhea (like a smaller, South American ostrich), and cassowary (with a bright blue face and what looks like a toenail on its head)."

Big Flightless Birds Come From High-Flying Ancestors (NPR)
"Scientists used to think all of these large, flightless birds descended from a large, flightless ancestor that lived millions of years ago when the southern hemisphere had a giant supercontinent. That supercontinent broke up - Africa drifted away, then India and South America."

Genomic Support for a Moa-Tinamou Clade and Adaptive Morphological Convergence in Flightless Ratites (Molecular Biology and Evolution)
"Phylogenetic analysis using standard homogeneous models and heterogeneous models robust to common topological artefacts recovered compelling support for ratite paraphyly with the little bush moa closest to tinamous within ratites."