It also requires most clients to be updated to support the new address system.
That's the killer: introducing Yet Another Type of Bitcoin Address takes a very long time and requires a lot of people to change their code. At least, that was the lesson learned when we introduced P2SH addresses.
I think it's just not worth it for a very modest space savings (10 bytes, when scriptSig+scriptPubKey is about 120 bytes), especially with the extreme decrease in security (going from 2^160 to 2^80 to brute-force).