But why should the hybrid system be the parallel hybrid and not the series hybrid system, basically a BEV with ICE as generator?

From a personal preference would like screaming engines over everything silent as nothing attracts attention more than noise. From one offs both Burton's 306 Cosworth and the NZ 207 Formula Renault come to mind. And of course the S1600/2000 sounded great but I am aware it wasn't exactly cheap.