For an operating system second only to Mac OS, and surpassing Windows, where did it go? How can it be so forgotten now?
GEOS on the Commodore platform faded out for a number of reasons:
The IBM 'Standard'
The PC platform had a larger capacity for upgrades and peripherals. The Commodore was already largely dated hardware by the time the 90s rolled in. With the growing complexity and power of applications, new hardware was needed - and the PC as a more modular system could grow with new innovations. However, the Commodore 64 / 128 were stuck in time, much like a games console, something that the C64 had become in the end.
Commodore's bad management
Commodore Business Machines had begun to lose its edge after the heady success of the Commodore 64. After several bad decisions, the company collapsed and filed for bankruptcy in 1994.
An OS dependent on the hardware
When GEOS was first created it was meant to be an embedded system (the Sky Tray). It was hand coded to utilise the processor to its maximum. You could not just write it in a high level language and compile for the hardware, it would take up too much RAM and would be slow. The 6502 processor was simple enough that a programmer could hand type the assembly code far better than any machine could. whilst this made the Commodore 64, and the 6502 in it, sing - it also meant that moving to a new processor architecture basically meant a total rewrite. This ruled out the Commodore 64 / 128's successor - the Amiga, which used a different processor architecture.
GEOS was not entirely out of the game though; the 6502 processor was being used in several other products at the time, and additional ports of GEOS were made. Most notably, on Apple's popular home computer the Apple II in 1988.