> Because DDR4 is electrically different and memory controllers are all on-die.
Them being on-die is exactly why you don't need a socket change to take full advantage of DDR4, since they directly showed a socket can support both at once. Unless you're particularly worried about people trying to buy a new motherboard for their existing CPU, but who does that? You can tell them no without blocking CPU upgrades for everyone else.
> pre-Nehalem
> AM3 CPUs would have both DDR2 and DDR3 memory controllers
Them being on-die is exactly why you don't need a socket change to take full advantage of DDR4, since they directly showed a socket can support both at once. Unless you're particularly worried about people trying to buy a new motherboard for their existing CPU, but who does that? You can tell them no without blocking CPU upgrades for everyone else.
> pre-Nehalem
> AM3 CPUs would have both DDR2 and DDR3 memory controllers
LGA1151 supported both DDR3 and DDR4.