Possibly to jump to PCIe 5.0?
MPX is really ye olde PCIe 3.0 + a bunch of proprietary internal Thunderbolt lanes and additional socketed power connectors, to avoid the clunky cabling you get with PC motherboards - having to plug additional power cables into GPUs and then externally plugging the GPU's displayport output into an input
on the same motherboard to then output over Thunderbolt, etc.
In theory PCIe 5.0's speed will obviate the need for an internal Thunderbolt bus, depending on how it's implemented, There's also
a new draft PCIe 5.0 power connector that can provide up to 600w for very power-hungry GPUs. It'd be nice if PC, power supply and GPU makers would copy Apple in creating a more cable-free power bus solution, but I'm not holding my breath.