Because Powerplay as it is needs to stand apart from the BGS, which is Powerplay 2.0 in all but name. The grindy mechanics become much more fun when you are evading others. I'm enthusiastic about that based on the brief glimpses of that happening in my time, and having that as normal rather than fleeting is something that would energise the feature.
I'm all for it standing apart. Now, for you maybe it becomes more fun when evading others, but it still doesn't change it being a grindy mechanic (and i thought the consensus around here was that grind is bad? And PP is as grindy as it gets!), but its not fun for everyone being jumped on by a wing of meta ships. And the thing is, you can already get that! If you are flying in open, then those who are playing the role of hunters are already in open! But how often are you actually having to evade others? In thoery, there is an imbanace at the moment between haulers and hunters right? Because all the hunters are in open but the haulers are split between modes. So you are at peak hunter right now. Thats it, that is max excitement you are getting. And when fortifying, exactly how much are you getting hunted, because when i was fortifying earlier in the week, i didn't see a single other player. Not even allies, let alone enemies. Where were all these mythical hunters? I presume in their own systems, because they weren't in ours.
Again, its fine that you want this, but you also want it to be more popular, so there has to be something change that will bring in a lot more players. Its questionable how many more people open only PP would draw in vs how many it would lose. But making changes just to please only those who already do it, without looking to bring in fresh blood, would seem short sighted to me.