The Knightfall books currently out are each about the size on an omnibus already. I can't see them reprinting them in the omnibus format now that those have been out for a couple of years as everyone interested in the story will most likely already have those 3 volumes. Not to mention that the contents would be more or less exactly the same with only a small handful of relevant issues missing from the paperbacks.

As for Bruce Wayne: Murderer, I don't think there is enough material there for an omnibus unless you also included Fugitive and even then, just like Knightfall, they are getting the comprehensive paperback treatment. Whilst I would have liked to have been both series in hardback omnibus format I just can't see it happening now.