Usually cost is a reason. Including a hardware chip for DSP tends to not be the cheapest thing in the world (a decent one is easily around the same cost as a GPU), and using special drivers for a feature currently in a small line of GPUs isn't a great use of money either.
Take the life of hardware DVD decoders in Macs... it lasted a whopping 3 products until the G4 hit, which closed the gap between software and hardware decoding (at least enough to not justify a 25-50$ chipset being added or sold as upgrades). The Lombard, the Pismo, and the B&W G3.
Already, H.264 decoding on recent Macs can do 1080p or get darn close to it. The main benefit of a H.264 chip is transcoding or encoding, which without a user scenario that makes sense (i.e... if users of an Apple app or peripheral can benefit greatly from it), isn't worth the trouble implementing.
I see this more being useful in Final Cut for HD authoring, or in iTunes for HD-DVD/Blu-Ray 'Managed Copy'.