It wasn't the ease of install or simplest-case-deployment, that's for sure. I did install it and kick the tires.
This was around the end of 2009, and i don't think there were many clients available (i see pycassa dates to 2010-04).
again, ease of development is much more important to us than performance; it'll probably be a long, long time before our db engine is the choke point, at which point we'll have resources to use a heavier tool.