I guess there's two reasons - the first is when they started this process it was before we had full electric vehicles and it wasn't clear what the best technology would be, at the time hybrid vehicles were very popular.

The second is that F1 has been carbon neutral for at least two decades with the FIA planting trees to offset the extraordinary amount of pollution that flying the F1 circus around the world generates. Even though reducing fuel does nothing to make F1 greener when you take into account the carbon generated by transportation it is important that the actual show is as green as possible.