My guess is that in the beginning it had something to do with VC's being desperate for an attractive, brilliant, successful female founder with world-changing ideas that could be slapped on magazine covers to show everyone that yes, they do invest in women. After the first round it's much easier to raise subsequent rounds, and the whole thing may have fed on itself - investors throwing good money after bad, both to save face and to chase the glimmer of hope that they were getting somewhere with this. Elizabeth Holmes has somehow kept the money flowing for a decade from extremely credible people, which means she is far better at placating investors in the face of slow progress than perhaps anyone in recent Silicon Valley history.