Why was the Sikorsky Firefly built but never flown?
The director of Sikorsky Innovations Chris Van Buiten called it a

technology demonstrator and research platform that will let us explore the technology, partnerships and technical collaboration...

On 2010 July 29 Verticalmag.com reported

First flight is anticipated later this year, upon completion of ground tests and safety of flight reviews in accordance with Sikorsky standard practice for all aircraft programs.

It vanished from the news a few months later.
Ten years later, it may be hard to get someone from Sikorsky Innovations to speak on the record about Firefly's abandonment, something that the company might consider embarrassing.  But it seems likely that some ground tests failed (cooling? software?), and either the partnerships with the companies providing the batteries, motor, software, etc., weren't important enough to the company (or its owner, Lockheed Martin?) to address those defects, or its other projects became more important.