Even though they are all w790 chipset motherboards, and compatible with both W-3400 and W-2400 chips, the motherboard can be configured without the capabilities that the W-2400 is not able to access, making it cheaper.
Motherboards tailored for W-2400 CPUs usually have 8 dimm slots, and cost around $700. They often have 3-4 x16 PCIe slots plus some other features. like 2.5g, and 8 sata ports.
Motherboards tailored for W-3400 CPUs usually have 16 dimm slots, and cost around $1100. They also have more fully enabled PCIe slots.
ie:
For the asus motherboards, to understand what is going on, you really need to see the block diagram for each motherboard which can be found in the manual under support documents.
$700 motherboard with $350 to $700 W-2400 CPU
$1100 motherboard with $350 to $700 W-2400 CPU
$1100 motherboard with $1200 W-3400 CPU
The block diagram for the Z790:
You can see that all of the chipsets have the same x8 DMI4. Which is essentially x8 pcie 4.