Why? Because not all powers are created equally. Sure, if you're a combat pilot for a power that's been heavily involved in combat and attracts a ton of combat pilots, forcing everyone into open sounds like a fantastic idea.
Suppose instead that you're Torval, Sirius or Mahon - powers that pretty much have absolutely no interest for combat pilots - why are you so intent on ruining their ability to actively ruin the fun that their players can have? They have no pilots capable or even willing to defend them, because the game rules have actively discouraged combat pilots from joining them.
Even better - why are you so intent on ruining four powers' ability to properly fortify? Torval, Delaine, Antal and Mahon all have inbound fortifications. That means that all of their fortification merits go through a bottleneck, namely their head quarters. If PowerPlay only existed in open, then you can effectively blockade those four powers, while the six others cannot be effectively blockaded. Sure, you can kill their fortifiers when they arrive in the HQ, but at that point they're not carrying any merits.
It gets worse. Suppose for a moment that Hudson and Mahon got into a prep war (happened a few cycles ago). Assume for a moment both powers have an equal amount of combat pilots. Which merit pilots are at greater risk - Hudson or Mahon? I'll break the suspense - it's Mahon. Mahon preps by carrying merits TO the preparation system (meaning they're bottle necked), whereas Hudson carries merits AWAY from the preparation system.
The game mechanics already give some powers an enormous advantage, and now you want the game to be even more biased towards those powers. I play almost exclusively in open, including when hauling merits back and forth, and I see absolutely no reason that solo or private groups should be barred from participating in PowerPlay, just like I see absolutely no reason that open should carry a bonus for PowerPlay participation, precisely because of the already massively unbalanced mechanics that are in play.
When I (Mahon pledge) fortify in open, I am taking a MUCH higher risk than any fortifier from Aisling, ALD, Patreus, Hudson, Winters or Sirius, because I have to bring my merits through a bottle neck. We all risk facing a rebuy screen, but Antal, Delaine, Mahon and Torval pilots fortifying are also risking a complete cargo load of fast tracked merits - which in my case works out to between 7 and 8 million credits. That's a risk that is never talked about, because no one (including Sandro and OP) seems to be completely oblivious to, and quite frankly I find it insulting to my intelligence whenever people bring up these topics, because time and time again, I have to point out that THE GAME MECHANICS ARE MASSIVELY UNBALANCED FOR EACH POWER. Currently that is something that can be handled, partly by cooperation, and partly by some people going into private groups. And that's without even bringing up the ability to fly in open without being able to actually instance with anyone - open solo-mode if you will.
If you want these changes, first get FDev to make the game mechanics identical for each power. Actually, if you can manage that, get them to fix the crap ton of bugs that already exist in PowerPlay before introducing new things that will inevitably introduce new bugs that can continue to muck things up for players.