Tom King had Omega Men, Vision, The Sheriff of Babylon and Grayson before he was given Batman. All of those were well received and DC decided to go with him as main Batman writer. For comparison, Snyder had American Vampire and around 10 issues of Detective before DC decided to go with him as main Batman writer in New 52. Obviously Snyder worked out better for them, but there wasn't anything weird in particular with King getting picked for Batman.
Also, from what I remember Mister Miracle had solid sales (something like 35-40k per issue) and was very well received so my guess would be that DC decided to permanently move him to maxi series and thats why he got Strange Adventures, Rorschach and Batman/Catwoman.