Right, I was just bagging on the killer B's because I find fun, but regardless, but a changing starscape to suit the specific area of space for a specific episode or set of episodes is a production cost that would get deleted in almost any show. Cool though it may be.
But it's not a matter of a single episode -- if they'd done it, it would've been for
every episode in the last two and a third seasons, because that's how long they spent near the Central Bulge. So you'd only have to make one set of new backgrounds and could use it for 60 or more episodes -- which is longer than many entire television series run! It would be a one-time cost that would be amortized over the rest of the series, and that's hardly prohibitive.
It's far from unusual for a series to add to its library of stock shots in later seasons -- like when TNG switched from the 6-foot
Enterprise miniature to the 4-footer, or when DS9 introduced the
Defiant, or when VGR phased out the miniature in favor of the CG model. The only difference is that they wouldn't be able to use the existing stock shots, but they could still use the original FX elements of the ships against bluescreen, or the original animation of the digital model; they'd simply have to composite it against a different background, which -- as I already said -- is no more complicated than matting an existing orbit pass against a different planet every week.
Besides, the whole nature of
Voyager dictated that they'd have to abandon parts of their stock library as the series went on. For instance, they couldn't use stock shots of Kazon ships after season 2, or Vidiian ships after season 3. So a degree of planned obsolescence was already built into the show's budget and logistics.
So there's no reason they couldn't have made this change if they'd chosen to. The will just wasn't there. Maybe it just didn't occur to them, or maybe they didn't trust the audience to understand a starscape that didn't look just like every other starscape they've seen in screen sci-fi for decades.