The main reason for adding hybrid was because the manus (Ford and Hyundai) demanded it to stay in WRC. More precisely demanded that the cars will be hybrid without caring about the hybrid specs at all.

The main goal of the rule set then done by FIA was to keep the car behavior as similar as without it, to be able to use the same petrol engines and to keep the costs on same level as 2017 cars (looks like the last part failed).

So the hybrid was added as a autonomous automatic unit that the drivers can't directly control, with a small battery and even more importantly strictly limited levels of energy use (limiting the amount of regen as well).
The goal was never to make the car as fast as possible or to make the hybrid radically change the driving.