Because David existed less to be a character in his own right than as a way to explore the loss of innocence in the kids and reinforce their paranoia. David forced them to confront their own brutality and humanity and pushed Rachel into a place of profound discomfort with herself; he removed the last hope the Anis had of trusting anyone; he pretty much sealed the deal on what they were willing to do to stay alive. He was a fine enough character on his own but his role was really to reinforce the motifs of the series, and once he did that there wasn't a heck of a lot he would have added.
He's more powerful because he only plays a major role once - his story is memorable and has a lasting impact despite his relatively brief appearance. David would have had to become a less intimidating villain if he'd shown up more often, because there's only so often you can have him appear unbeatable and scary and still get away - look at V3's villain decay versus V1's. Was Taylor scarier the second time around? Was Crayak? David certainly wasn't, and I doubt even in a different scenario he could be because they already got away from him before despite him having the upper hand in just about everything. It undercuts how seriously you can take him the next time you see him.
Admittedly, I'm not hugely fond of David, but the David Trilogy is one of my favorite parts of the series because she uses David really well to highlight the dehumanizing effect of the war, and David makes a heck of an intimidating villain.
