While some lawyers do occasionally file lawsuits based mainly on their targets having "deep pockets", like Microsoft had, this is not generally successful. In order for a typical corporate lawsuit to gain any traction in the court, the plaintiff must provide sufficient "standing".
In order to establish standing, Apple sought to convince the court that Microsoft's actions violated an earlier licensing agreement for Macintosh technology that was inked between Apple and Microsoft in 1985. Furthermore, HP was included in the lawsuit because it was alleged Microsoft had sub-licensed Windows technology to HP for NewWave.
The litigation arose out of a dispute whether an earlier version of Microsoft's software, Windows 1.0, infringed Apple copyrights. Microsoft and Apple sought to put that dispute to rest by an agreement on November 22, 1985 ("1985 Agreement"). By that agreement, Apple granted to Microsoft a non-exclusive license of the audiovisual displays in Windows 1.0. As the court found on March 20, 1989, however, the 1985 Agreement is not a complete defense to this action, because it was limited to the visual displays in Windows 1.0 and did not cover displays in Windows 2.03 that were not in the prior work. Apple Computer, Inc. v. Microsoft Corp., 709 F. Supp. 925, 930 (N.D.Cal.1989).
Given that no such prior licensing agreements existed between Apple and Commodore-Amiga, it is apparent that the same complaint could not be brought against them by Apple. If Apple wanted to file subsequent lawsuits against Commodore, and others who used similar GUI elements to the Macintosh, they really needed a victory against Microsoft and HP first, which would set a precedent about copyright protections applying to the Mac GUI. After all, copyright was the underlying basis for these Apple claims, not stronger protections, such as registered U.S. patents.
Since Apple ultimately lost the copyright argument, there was never any point to reviving this case by going after other alleged "infringers", such as Commodore. The court had already ruled no infringement had taken place by Microsoft, so Commodore was inoculated against such a claim.