[1998 voice] "why do I have to use EPP mode if I want my scanner to work through my PCs parallel port?"
Because there's no such thing as a "parallel port scanner", at least by the implied meaning of "the de facto standard established by the printer port on the original IBM PC." Parallel signaling is a general EE concept, and indeed EPP and ECP do fit that definition, but that's about it. They use completely different pinouts and are essentially distinct standards that just happen to be overloaded onto a single DB25 port on the PC.
The PC was absolutely starved for external peripheral interfaces for it's first 15 years before USB came out. Unless you bought a SCSI card there was simply no way to get data in and out of a PC without a custom ISA interface. Serial and parallel were sort of good in 1981, but absolutely dog slow by the standards of just a few years later. A true parallel port scanner would be limited to around 50 KB/s; unusably slow. The fact that nobody ever replaced these ports with anything better before USB has always bothered me.
It turns out they did. EPP could transfer at two megabytes per second, which is in fact faster than USB 1.1, and was not burdened with silly shit like a dedicated paper end signal. But I guess Xircom et al were obsessed with not adding another port to the PC, so they shoved it through the same dumb oversized 25-pin D-sub connector and called it a "mode" even though it was a completely unique and unrelated interface.
This eliminated arguably the biggest advantage of USB, the tiny and unbulky plug, and also left an enduring legacy of confusion and incompatibility. Because virtually every PC had just one parallel port, expanding its capabilities forced people to use peripherals in a de facto bus topology through stacking connectors, and often you had shit like a printer that would only operate in SPP mode and a scanner that would only operate in EPP mode, so you simply couldn't use them both on the same PC without rebooting every five minutes.
It seems like this all comes down to an inability to see the coming future. USB was basically the death knell of parallel signaling, I don't think we ever did it again after that; every interface became a differential serial pair, and even in the cases where there are multiple (PCIe) they're treated as distinct interfaces. A PCIe x16 graphics card is (or at least can be) 16 separate serial interfaces to the CPU. Was this sea change due to a shift in technology, or a shift in thinking?
