The LC 475 supports 24 bit addressing and will work with the IIe card *if* it's running MacOS 7.5.5 or earlier. It was MacOS 7.6 and later that removed support for it.
As to why an LC 475 can't run System 6 while (in most cases) newer PCs can run ancient operating systems it's an apple to oranges comparison. In the PC world there's a basic set of hardware functionality, collectively dating to about 1987 or so plus a few later extensions, which all newer machines are expected to support at the bare metal level. (The ridiculously powerful Nvidia card with a gig of VRAM in your brand-new gaming PC still has buried in it the ability to emulate an IBM PS/2's 256k VGA card, and that's the mode it boots up in once the BIOS splash screen is out of the way.) There are holes in this model: for instance, there's no universal network device that newer cards emulate, which means you need a hardware specific driver if you want the ancient OS running on your Intel i7 to be able to surf the web. (Which will probably be impossible to find.) But for the *most part* the hardware and BIOS support will be there and compatible enough to get the OS running to some degree.
The classic MacOS, on the other hand, is a diabolically schizophrenic pile of code in which many of the core functions of the OS actually exist as routines in the computer's ROM. In *theory* if all the hardware-interacting parts of the OS were in ROM and the APIs were completely standardized this would work to make MacOS *more* hardware independent and forward compatible, but in practice Apple didn't leverage that very well. As executed, a release of the Classic MacOS needs to recognize the machine it's running on so it will select the correct code paths, either to adjust itself to the capability of the hardware or enable patches for known bugs. That's why Macs have that "Gestalt ID" system.
There are some instances where "trivial" hacking an OS will enable a machine with an unknown/disallowed Gestalt ID to run it, with various degrees of success. But System 6 on an LC475 is a pretty huge jump. On top of ROM changes (the Quadras have larger ROMs than the Mac II series) Apple officially didn't support the 68040 CPU until System 7. And unlike Intel CPUs Motorola's were usually *not* complete supersets of the earlier models. The 68040 is missing several instructions compared to a 68030+68882 combination, and because of some changes in how the prefetch/caching mechanism works there are several areas of incompatibility related to self-modifying code that are "barely legal" on the 68030 but cause crashes on the 68040. There were third-party upgrades for Mac II-series machines *would* run System 6 on the 68040, but the enablers for those upgrades include patches for several broken areas within the OS.
So... it probably would be *possible* to patch together a working System 6, probably by incorporating code from an accelerator card extension, but the fact that apparently no one has done it yet is sufficient evidence to say that doing so would be "hard".
As to why an LC 475 can't run System 6 while (in most cases) newer PCs can run ancient operating systems it's an apple to oranges comparison. In the PC world there's a basic set of hardware functionality, collectively dating to about 1987 or so plus a few later extensions, which all newer machines are expected to support at the bare metal level. (The ridiculously powerful Nvidia card with a gig of VRAM in your brand-new gaming PC still has buried in it the ability to emulate an IBM PS/2's 256k VGA card, and that's the mode it boots up in once the BIOS splash screen is out of the way.) There are holes in this model: for instance, there's no universal network device that newer cards emulate, which means you need a hardware specific driver if you want the ancient OS running on your Intel i7 to be able to surf the web. (Which will probably be impossible to find.) But for the *most part* the hardware and BIOS support will be there and compatible enough to get the OS running to some degree.
The classic MacOS, on the other hand, is a diabolically schizophrenic pile of code in which many of the core functions of the OS actually exist as routines in the computer's ROM. In *theory* if all the hardware-interacting parts of the OS were in ROM and the APIs were completely standardized this would work to make MacOS *more* hardware independent and forward compatible, but in practice Apple didn't leverage that very well. As executed, a release of the Classic MacOS needs to recognize the machine it's running on so it will select the correct code paths, either to adjust itself to the capability of the hardware or enable patches for known bugs. That's why Macs have that "Gestalt ID" system.
There are some instances where "trivial" hacking an OS will enable a machine with an unknown/disallowed Gestalt ID to run it, with various degrees of success. But System 6 on an LC475 is a pretty huge jump. On top of ROM changes (the Quadras have larger ROMs than the Mac II series) Apple officially didn't support the 68040 CPU until System 7. And unlike Intel CPUs Motorola's were usually *not* complete supersets of the earlier models. The 68040 is missing several instructions compared to a 68030+68882 combination, and because of some changes in how the prefetch/caching mechanism works there are several areas of incompatibility related to self-modifying code that are "barely legal" on the 68030 but cause crashes on the 68040. There were third-party upgrades for Mac II-series machines *would* run System 6 on the 68040, but the enablers for those upgrades include patches for several broken areas within the OS.
So... it probably would be *possible* to patch together a working System 6, probably by incorporating code from an accelerator card extension, but the fact that apparently no one has done it yet is sufficient evidence to say that doing so would be "hard".