I suspect a big factor was PSU design. Even large OEMs contracted it out, and there probably reached a point where manufacturers started to offer catalog items

I suspect a big factor was PSU design. Even large OEMs contracted it out, and there probably reached a point where manufacturers started to offer catalog items in the 200-400 watt range where the primary size constraint was the 80mm exhaust fan. Once that's the constraint, it's going to look a lot like LPX, and you get a reinforcement factor around specific dimensions when vendors say "If we use the exact same screw positions we can compete as a drop-in replacement" It's interesting that PCs adopted the LPX PSU but not the cases and motherboards. I had always seen LPX described as a bit more proprietary than (baby-)AT designs-- you could fit anyone's mainboard in a generic AT case, but even a HP "LPX" mainboard and riser card might not fit in a Packard Bell case.