For many players, the main reason they wanted dual spec was to be able to switch between "leveling" and "grouping" specs, and obviously if you're already max level, you won't have much need for a leveling spec any more (more likely you'd be switching between "solo" and "raid" or "PvP" specs). Ghostcrawler's stated reason for requiring a max level was that they didn't want lower level players to be confused by having access to more than one talent spec too early, but as Zarhym says, Cvarto makes a good point: if a player uses only one spec for 80 levels and then gets the dual spec feature, he/she may not have enough experience with different specs to have any clue about how to choose another one.
The standard response to that seems to be that you don't need different specs to level up -- most players say that regular instances can be healed or tanked even with off-specced characters. But it's true that having no change at all in the way your specs are laid out between level 10 and 80 is quite a gap -- it's hard to believe that putting the dual-spec feature in at, say, level 60 would confuse people all that much.
Blizzard is keeping their options open here -- there's no way we're going to have two different specs to choose from right at level 10 when talent points first come into play, but it seems pretty likely that they may reconsider requiring level 80 to be able to switch between two different spec choices.