Spiking that story would have delayed Theranos's collapse a bit (it isn't like if one news organization doesn't report a story no one else will ever find it), but had Murdoch spiked the story and unloaded his stock while it was still worth a lot, he could have been in legal trouble for trading on inside information, something only sources within the company knew.

This kind of spiking backfires badly when discovered (NBC blocking Ronan Farrow because they had their own related problems, he wound up taking the story elsewhere).