Memory hard PoW doesn't prevent specialized hardware from taking over. Witness the ASICs for scrypt, Equihash, CryptoNight, ethash, and Cuckatoo. Also, a good PoW should be trivially verifiable and thus hashcash makes a poor memory hard PoW [1]

[1] http://cryptorials.io/beyond-hashcash-proof-work-theres-mini...