I don't know for sure, but it speaks volumes about the popularity of powerplay 1 if one of 11 powers is so poorly supported. I can't imagine them being much more popular with PP2. As you say, other powers are bigger, by far. I'd be interested in knowing what "by far" actually means. Are we talking tens of regular players? Hundreds? Thousands? Unlikely considering the size of the player base in total.
Some powers are simply small because they don't appeal to people. If you join Power discords you can get a decent idea yourself, and map power progress to what players are tasked with.
How many people regularly play ED? A few thousand? Ten thousand? 20? 30? How many people are interested (or would be interested) in PP, either mixed mode or open only? Are we even talking double digit % of players? 50% or more seems a stretch. Even 20% might be pushing it.
It depends on how you look at it- those who play to push the strategic metagame and those who want rewards. V2 has great rewards (probably the best out of the game and ongoing for 'free') so this line will blur (especially so when any activity counts). Given too that module access is linked to being pledged (as in, buying more) you might have many more players- at least until they reach Tier 11 care package level.
With PP2, even getting a couple of thousand players regularly engaging over 12 powers, you're not looking at huge numbers. I'd guess less than 100 supporters of a particular power on at any given time of the day, much worse for an unpopular power.
I suppose based on that, i can understand why you'd want everyone in open. Even if it was, chances of meeting another player from your own or an opposing power is still going to be pretty slim unless its a busy system.
Powerplay also has areas of interest to funnel players- V1 was the control system, capital and expansion. V2 also has them, plus strongholds as well as organically arising pressure point systems coupled with tug of war (and not 100% and done) mechanics. So if we were talking about being spread over the entire galaxy I'd agree, but its not. Established power groups will still push, the only difference is you'll have other non-aligned players elsewhere too.
Finally, keep in mind, that while open only might attract some players, it might turn off some players. Then keep in mind which sorts of players open only would attract and which sorts of players it would turn away.
The polls that asked the question saw that those lost would have been replaced twice over. In the end if its open / weighted or not, the feature has to be
consistent otherwise it will end up like V1.