"Any sufficiently advanced technology is indistinguishable from magic."
(A.C. Clarke)
In this case, the investors just followed the money, assuming someone else had checked the magic. Holmes was happy to lead them along.
Technology will continue to give us great advances in medical diagnostics and treatment, but it takes a lot of work and time to make each little step. Theranos promised it all at once and right now -- that's usually not how science progresses.