Because DDR4 is electrically different and memory controllers are all on-die.
Intel could get away with doing that pre-Nehalem because the memory was connected via the northbridge and not directly (which is what AMD was doing at the time; their CPUs outperformed Intel's partially due to that), so the CPU could be memory-agnostic.
AMD would later need to switch to a new socket to run DDR3 RAM, but that socket was physically compatible with AM2 (AM3 CPUs would have both DDR2 and DDR3 memory controllers and switch depending on which memory they were paired with; AM3+ CPUs would do away with that though).
There were some benefits to doing that; the last time Intel realized them was in 2001 when RD-RAM turned out to be a dead-end. Socket 423 processors would ultimately prove compatible with RDRAM, SDRAM, and DDR SDRAM.
> 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
Intel could get away with doing that pre-Nehalem because the memory was connected via the northbridge and not directly (which is what AMD was doing at the time; their CPUs outperformed Intel's partially due to that), so the CPU could be memory-agnostic.
AMD would later need to switch to a new socket to run DDR3 RAM, but that socket was physically compatible with AM2 (AM3 CPUs would have both DDR2 and DDR3 memory controllers and switch depending on which memory they were paired with; AM3+ CPUs would do away with that though).
There were some benefits to doing that; the last time Intel realized them was in 2001 when RD-RAM turned out to be a dead-end. Socket 423 processors would ultimately prove compatible with RDRAM, SDRAM, and DDR SDRAM.