Harry Potter and the Prancing of Ponies

The inventors of the Mirror wanted to create a device that could channel infinite magic and grant wishes. The hard part was making sure those wishes would never destroy the world. Showing a person their heart's desire is a sensible precaution to put into a wish-granting machine designed NOT to go horribly wrong.

The Atlanteans got close, even managing to imbue some amount of intelligence and morality into the Mirror. But they never finished their work; the Mirror never DID get to the point where it could grant wishes... at least, nobody believes it ever got to that point. But then, nobody has ever been trapped inside the Mirror and returned to tell the tale.

When the Mirror traps someone inside itself and outside of Time, that person and their consequences can no longer affect the world. In that circumstance, with a person fully under its power, the Mirror isn't limited in what it can do. Sometimes, it can even grant seemingly incompatible wishes.