Because "Horton Hears a Dalek Shouting 'Exterminate!'" just didn't roll off the tongue well enough.
I can see all the best-selling titles now: "King Learax," "A Midsummer Night's Grinch," "Othello, the Places You'll Go," "Yorick the Turtle and Other Stories," "Fox in Saxony..." the possibilities are limitless! - Matty Malaprop
Because "Da Mystery of Chessboxin'" is on par with "The Lorax" in terms of intrigue and awesomeness. - Matty Malaprop
Those wee little Whos were probably just figments of his imagination, but Horton swears he won't get fooled again by them. - Matty Malaprop