1. Sculpts are mondo expensive to tool, and when a company can't recoup the costs with a basic body that can be re-used, repainted and generally re-released, it drives the cost through the roof. The basic bodies on human characters require almost no tooling, just head sculpts, so the cost to produce the figure is minimal. The outfit costs are negligible, really the only cost to the company is the salary of the people involved in creating the figure and the licensing and distribution costs. It costs almost nothing. Tooling's been done, toss a robe on the thing and release it again. It's the best case scenario. With figures like clones and troopers, SS can release repainted versions forever and still recoup costs. But aliens are one-time only things, and new tooling does not get spread out over many releases. It drives the price way up. Momaw Nadon and Bossk had just a few add-on parts to a basic body, and cost a fortune. At least Figrin D'an could be issued as various Bith characters with the hopes that SOMEBODY would buy a handful of the guy. The Rebo band would require entirely new sculpts across the board, and nobody would ever buy more than one. Maybe Rebo himself and Droopy could use the Gamorrean Guard body as a starting point, to save on some tooling costs, but overall, these would be the single most expensive figures SS ever did.
2. Tertiary characters who don't have an "action" role are a tougher sell, fewer people are interested in them. Even the "casual" collector, to the extent there is such a thing, might pick up just Jedi Luke or Darth Vader. But only the die-hards are going to get a musician.
3. Even if somehow Sideshow found a way to justify the expense of one-time only, tertiary, non-action characters, the problem still remains that these figures only really "work" as a set. Hardly anyone is going to want JUST Max Rebo or JUST Sy Snootles, which means not only do you have obscure non-action characters who retail for about $300, but the appeal of getting them plummets for anyone who doesn't want to get at least four or five of them (Droopy, Max, and Sy are a bare minimum, with Joh and Dodah rounding things out). That means people have to decide between dropping over a thousand dollars to get a decent band together, or dropping only $3-600 to get a random figure that doesn't really work on its own. The number of people interested in these just got even slimmer.
It's barely worth considering, IMO. It would be one thing if SS were running out of ideas or ways to milk the license, but when there are SO MANY far more popular, easier ideas out there, this one would be just a way to try to kill the line.