Slots 1, 2 and 3,4 aren't available because the hardware doesn't support them.
Slots 1, 2 and 3,4 aren't available because the hardware doesn't support them.
It's been a persistent issue. If a dual-channel setup existed, the memory should be positioned right next to each channel.
It seems the issue might relate to the motherboard's layout. Since you're unfamiliar with this topic, someone else would need to clarify it for you.
It's just their design. Board manufactures can be placed together without any real restrictions, as long as the setup makes sense. The main reason I see for not doing it is probably to improve airflow when using dual channels with two DIMMs. For example, the Gigabyte GA-MA785GM-US2H supports dual channel by using side-by-side slots.
Simpler to connect the PCB this way. A single trace path serves both channels 1 and 2, while another handles channels 3 and 4. Mixing them up would create a complicated routing since each signal destined for the first slot must detour around the second to reach the third.
The wiring for that would be much worse. Connecting modules on the same channel uses the same wires, which simplifies things. Placing slots next to each other on the same channel helps too. As you add more wiring, keeping signal quality becomes more difficult.
because traces if you want the channel 1 dimm apart, one dim would be much nearer than the other to the same channel, and that can lead to stability problems too, side by side makes it simpler to map the traces, if you've ever tried drawing pcb traces you'll understand how tough it is, here are two videos that fit your topic