The difference is that all the Dylan "bootleg" material was owned and recorded by Columbia (now Sony) (and I know there were some certain exceptions to this, such as the Witmark release, but by and large this is true). The Miles material may well be radio recordings, audience recordings, and the like, that were never in Sony's vaults. It'll be interesting therefore to hear the audio quality of what they'll be releasing.
I am sure there are multiple sources for some of this Miles Davis material but between Sony and the estate of Miles Davis there must be a certain amount of unrelaesed material that the already own that could be made available commercially.