Why did Heart Aerospace change their all electric airliner to hybrid electric?

When I talked to Ben Stabler at Heart Aerospace, it finally clicked for me why they scrapped the all electric ES-19 design for the "independent hybrid" ES-30 - its all about IFR reserves and regional airliner economics - and it turns out those turboprops might not be used much... but they're a mass savings, not dead weight. How?