Commodore, softwarepublishers and Doom killed the Amiga
When the Amiga was introduced in the mid-eighties, it was a kind of revolution with its crystal clear sounds and hig-resolution graphics. The Amiga was a revolution. It was lightyears ahead of the consoles and PCs in that time. Softwarepublishers immediately produced ground-breaking titles like Marble Madness and Defender of the Crown which were much better, smoother and prettier than on other platforms. For Joe Public the Amiga became the machine one must get for these titles alone.
I agree with 7-Zark-7 that Commodore's R&D efforts weren't that great in the Nineties. In 1992 there were rumours about a brand-new Amiga family with AGA-capabilities: the A1200. I was shocked when I found out the soundchips weren't improved and the AGA display could 'just' match the VGA-palette that was common on PCs those days. The A1200 was more an evolution in the Amiga family. This time the softwarepublishers did not take up the task in producing ground-breaking titles for the AGA-machines. Their business-rule was simple: the old Amigas (A500, A2000) had a much larger userbase, so they would get much more profit on these machines. The result: they just churned out AGA-updates of existing Amiga games and if an 'AGA-specific' title was produced it could be easy converted to the old Amigas.
Around that same time Doom, a new groundbreaking experience, was released on PC. It astonished Joe Public, so they chose that machine. It was impossible to convert a game similar to Doom in a short time to Amiga. The game proved the Amiga was laying behind in this new 3D era. It turned to be the final nail on Commodore's and Amiga's coffin.