Mainly because using more sticks stresses the IMC on the CPU more, which means you will need more CPU VTT (or QPI/DRAM depending on bios terminology) voltage. In general it stresses the CPU more too, so only running 3 sticks as opposed to 6 does indeed give you some higher headroom to OC.
Also, for reference it is DIMM not DIM.
Also, for reference it is DIMM not DIM.