USB was intended to replace PS/2, Serial, and Parallel ports. Remember those parallel port scanners that were slow as hell and froze the PC while scanning? These were terrible.
In order to succeed in a commercial environment where every cent matters, USB had to meet stringent requirements:
For mice and keyboards, which were already very cheap devices at the time, it could not add any cost. This means it would have to be implemented inside the already dirt cheap microcontroller in the mouse/keyboard: pretty slow devices with as little RAM and ROM as possible, manufactured in a process that doesn't allow for anything fancy.
For more expensive peripherals like scanners, printers, USB storage, etc, the case is less clear cut: replacing the kludgy and expensive parallel port with much simpler and cheaper USB would already save costs, but then USB would have to be compatible with the microcontrollers used in these types of peripherals at the time, so a speed of 12Mbps seems adequate.
In addition, to get a wide adoption of the standard, motherboards must carry a lot of USB ports. At the time, during the transition, motherboards would offer both parallel/serial/PS2 and USB at the same time, so the cost per port must be very low in order for it to succeed. USB is usually implemented in the chipset, which is manufactured in processes allowing for much faster and more complex chips that a mouse microcontroller, but still, bleeding edge speed would add to the cost.
This drove the design of the entire protocol, which assumes an extremely dumb device that does what it's told.
Ethernet and FireWire are much more costly. On the software side, being actual networks (not master/slave like USB) they use rather complex protocols which require fast microcontrollers with lots of memory. It is possible to implement USB1 on a 8-bit micro with 64 bytes of RAM that costs cents, but for FireWire/Ethernet you would need 16-32 bit micro with 8-16k RAM at least, and much more ROM too. It's not economically feasible to implement these in a mouse.
On the hardware side, USB1 is dead simple due to being slow. Ethernet could have been made simpler and cheaper with a length restriction to ditch equalization, but there is still a lot of hardware complexity in the MAC/PHY. Plus it uses much more power due to being designed for long cables, which requires controlled impedance, etc. And FireWire is full duplex, so it needs two more wires! That will add cost to everything...
Basically for this type of standard to succeed, it needs wide adoption, and for that, cost matters a lot more than performance. Cost is the main reason why FireWire failed.