[sliken]

I believe it's just the advantages you get from very short trace lengths. Dimm slots are usually inches away, so you end up with long traces from CPU -> dimm slot, pay the overhead of the dimm slot connection, and then traces within a dimm.

LPDDR on the other hand move the individual dimm chips as close as possible to the CPU and don't have any connector. This also makes it much easier to have wider memory. A 13" MBP can have a 512 bit wide memory system with at least 16 channels in a thin/light laptop that is quite power efficient. To get similar with DIMMs you'd have to buy a dual socket server motherboard with 8 channels per socket and would be lucky to fit that in an ATX size motherboard in a 1.75" thick chassis.