While I don't doubt that all of the reasons they give are strictly true, I wonder if they dropped CC and other public licenses because to host such an image still costs money but they don't get any revenue for it. So if the percentage of images that are holding non-revenue generating licenses is too high, their fixed costs cut too far into their revenue to sustain operations.

One could argue that the might work on being more efficient about their fixed costs but this seems a simpler fix.