Likely because his definition of a hard and soft reboot are different from what you hold them to be. As it was he used stuff that had already existed and did something different with it, which would make it a soft reboot. A hard reboot would have made Diana an alien spy from the planet Zog for the goo people and he knew that would not fly with anyone.
Which he did, while some people get caught up on ultimately unimportant details.
Nothing Finch did sat well with anyone, that run was an abomination on so many levels; art, writing, story, characterization and everything in between.While I liked Finch's run, she was editorially mandated to bring Donna back as a villain which didn't sit well in a lot of reader's eyes. Plus, as much as I liked the Finchs' run overall, their revamps of Aegeus and Dr. Poison were lackluster to say the least.
Now DC can jettison the savage Amazons, adulterous Hippolyta, creeper Orion, Hecate-created Donna, Russian Dr. Poison, and spoiled brat Aegeus and get back to work on making Wonder Woman awesome.
Could have done that with a tweak here or there, a reboot was neither wanted or needed. Especially not when you have to invent a problem then solve it that just happens to take the established setting away.