They've probably moved it forward in order to terrorise more young children, because it got a Fear Forecast Factor of 5 ("scariest episode of Doctor Who yet") from the reviewing panel of children the BBC have introduced. The imagery of the series will be indelibly imprinted in their tiny brains, forcing them to purchase plastic memorabilia, cash-in books and memory cube recordings in later life in order to stem the compulsive nightmares. Mary Whitehouse would not have approved!
Less imaginative conspiracy theorists might just think it's something to do with scheduling to accommodate the Eurovision Song Contest.
