Surely the parent company, Commodore, going bust was the killer?
Admittedly the Amiga would have fared a lot better had Commodore marketed it better, but any platform is pretty much doomed when the company goes bust.
Of course there is the question as to what killed off Commodore, but I'm not sure that it's clear that it was because of Amiga related things. The Amiga may have started to fall behind in 1993, but I've heard arguments pointing out that the Amiga was still selling well back then, and it was more due to problems with their PC product lines.
If piracy had any effect, it was that PC/Mac had CD-ROM (which at the time was pretty much unpiratable), where as the Amiga was still on floppies. But as others have pointed out, piracy can actually help sales of hardware, and there's been plenty of piracy on other successful platforms in more recent times.
