Ironically, it is a very simple reason: a creature affected by Polymorphic organism it must have a CR or level greater than or equal to the Challenge Index of the creature in which they are being transformed; and the player characters have no challenge classifications. And since they do not have the same statistics on whatever they have been transformed, it is not appropriate to simply paste the CR of the form into them.
Therefore, a druid who has taken the form of a black bear, without a challenge rating, would not be eligible to be the type of animal to which an objective creature Polymorphic organism could become
The main restriction in Polymorphic organism it is "a beast with CR equal to or less than that of the target", without the same stipulation found in Wild form That has to be a beast that the caster has seen before. Because the Druid NPC has a CR, and presumably it was calculated with alternative forms that it could take into account, the real questions are whether
- The Druid NPC counts as a beast while in that form, and
- the Polymorphic organism the spell allows the target to polymorph into a "specific" version of a beast, and not just the platonic ideal of a beast, and
- The changed-shape Druid NPC represents a specific version of a beast, and not its platonic ideal
For the first question, I think the answer is yes, in fact they count as a beast, given the answers to questions that are based on a similar logic.
For the second question, I think the answer is also yes, it allows the target to become a specific animal. There is no text that contradicts this, and even the specific non-demicanonic beasts found in the Adventure Modules have a Challenge Classification associated with them, and are generally valid targets for Wild Shape and Polymorphic organism.
Then the last question remains: does a Druid NPC changed form represent a specific version of the beast whose form they have taken, or do they represent a generic version of the beast whose form they have taken? And I think the answer here is also yes: they represent a specific version of the beast. They are a beast with the same statistics as the form they have taken, except with (presumably) higher mental statistics.
As far as I'm concerned, it's legal Polymorphic organism a creature in a druid with a changed form or with a wild form (NPC), and in doing so, the polymorphized creature will inherit the mental statistics of the druid.
At my table, because of this decision, I think I would extend that to allow this to be valid if the Druid is a player character; but as I said, I don't think it's valid according to the rules as they are written.