The interface from a PS/2 keyboard to the CPU is quite direct, requires almost no software, and is well documented. Its hardware implementation made it easy to fool, as it happens when a KVM switch is used. It was just perfect at the time it was designed. These were the MS-DOS days. Don't think of multi-task on a PC, or internet security, in 1984. They were already a good evolution from the serial mice.
Its almost direct-to-CPU connection became a liability in a context of modern operating systems, which have to control every I/O to ensure security and stability.
One of the USB goals was to allow the replacement of all the peripheral ports that were connected through the original ISA-bus of the PC. The USB interface hardware is cheaper and offers a better speed than the PS/2 Mini-DIN. The USB hardware also contains circuitry to detect the presence of the peripherals, to detect overloads. And atop of that is a complex software stack and protocol that is handled by the host computer. Fooling a USB port with a KVM switch is a much tougher challenge than a PS/2 port.
The Firewire port has some definite hardware advantages over USB, but it was not designed from the start as a general purpose serial expansion bus. It was first intended as a digital link between video equipment. It did not all that was needed to integrate easily into a complete computer environment. Another reason why Firewire did not take the market is that it was, in the computer world, endorsed by Apple. Behind the USB effort was Microsoft at the main seat. That explains several things.
With Microsoft behind USB, they may be among the first to eliminate PS/2 versions of their mouse and keyboards to set the example.
KVM switches will improve in the future, they have no choice. But meanwhile, is Microsoft the only one to have a keyboard layout like the natural ergonomic?
