Blizzard made leveling longer to make it so new players might actually consider playing WoW.
Ever leveled through a toon before the changes? The gameplay was complete and utter ass. There was no enjoyment in it, no skill or thought ever went into it, you didn't even learn what a rotation was. It was just a slog you went through to have a max level character, which I can tell you in my case is why I couldn't be bothered to level more than one alt before now.