Affordability was tested
One of the many tests placed on these homes involved their affordability.
The Illini home, for instance, has an interesting design I linked to. The structure is based on a series of arches, made of laminated bamboo, both cheap and light. The arches hold the structure, in the same way conventional structures are framed-up.
Laminated bamboo is something I had never heard of before. But it's highly renewable, and can be made reasonably strong.