It's a good question.
Particularly Neelix and Seven had duties that placed them senior in the hierarchy than your average crew member, it would make sense that they should carry some kind of provisional rank to give them the authority to perform the duties as needed. Neelix was the de facto quartermaster, and Seven became the acting head of a minor department (astrometrics) and so they would need to interact with the crew on any number of administrative issues, and would need to pull rank sometimes.
However, I think the main reason why the show never directly showed this was to underline the fact that Voyager is both an explorer and a safe harbor, happily accepting new people, but not enforcing their own values onto them. While Kes, Neelix and Seven became part of the crew, but retained their civilian status to match their tendency to be more independent than the rest of the crew.
Kes and Neelix leave eventually, and Seven threatens to several times, which wouldn't have really been possible if they were in uniform. In this regard, the lack of rank is more of a character trait and an obvious way to set them apart from the crew rather than a choice which makes sense against the rest of the backstory. The fact that none of them need official rank to get what they want also tells us a lot about who they are. They each prefer their own approach to achieving their goals umong the crew (charisma for Neelix, good natured optimism for Kes, fear and occasional violence for Seven). It's simply not in their natures to ask for some artificial means to back up their personalities.
By contrast, all of the Maquis crew (grudging as they may have been) were alpha quadrant natives and presumably were happy enough to be going home, and at some point realised that they had a need to work together with starfleet for personal survival.
From Janeway's point of view, her ship needed a crew, and many of the Maquis had relevant experience, and so press-ganging them into starfleet would underline her authority as well as serve to prevent them from simply leaving and settling on a new planet, leaving Voyager dangerously undermanned.
Given the rather dire circumstances and that the Maquis came from their own hierarchy (and many of them from starfleet before that) simply being put in uniform and in a familiar pattern likely went a long way towards getting the Maquid crew working well and ensuring their continued loyalty. Also, it broke down the differences between Maquis and Starfleet and greatly eased the tensions between them, becoming part of what made Voyager greater than the sum of its parts.
It makes sense to give the majority of the crew ranks, because there needs to be order for the ships critical functions. For these three, while they were useful, they worked off their own initiative and were never likely to be directly in charge of anyone, which is probably why Janeway never felt the need to give them even a provisional rank.
Kes and Neelix were never in roles that would require them to be in the chain of command. Seven was not shy about barking out repair orders from time to time, but officially she never rose above crewman status -- perhaps because of her clashes with Lt. Torres, or perhaps because she was too disinterested in Starfleet inefficiencies to request a higher rank.
I think the answer to the question "Why weren't Kes, Neelix or Seven of Nine not given provisional Starfleet ranks like the Maquis crewmembers were" is in the question itself. That is, the reason Kes, Neelix or Seven of Nine were not given provisional Starfleet ranks like the Maquis crewmembers were is because the Maquis crewmembers were given provisional Starfleet ranks.
A lot of the bullshit Janeway went through in the first season had to do with the political leanings of the Maquis. Under the circumstances, Janeway had no choice but to try to integrate the Maquis into her command. Some, like Chakotay and B'Elanna, were "almost" Starfleet. Chakotay was even a Starfleet "grad" but was thrown out, if memory serves. It is reasonable that the last thing Janeway would have wanted to do, given the lingering politics of the Maquis, was to swell their ranks by attributing any more of a cohesive sense than they already had; that is, having already given "provisional Starfleet ranks", first of all, sends the message that Chakotay and crew were not Starfleet, but, second, also does not "reward" them by giving them new "members" in the persons of Kes, Neelix and Seven who would then seem to be viewed in the same way, by "Starfleet". Conversely, Kes, Neelix and Seven would not have benefitted from nor would they have wanted to parachute right into the middle of the Starfleet-Maquis conflict already set to stage.
It certainly makes you long for the harmony of good ol' NCC-1701-D. But, ST:VOY was a vehicle for a whole new bunch of stories, so what the hell.