I remember early in Retail, people complained to Badspot about the Ctrl+z limit being too low. He said that the game has to remember information for all the bricks so it knows what to kill.
A Ctrl+Y would be out of the question, it'd have to remember more then a brick to delete. It'd have to remember the colors, effects, states, events, etc. Way too much information to store, especially when you may not use it.