How about this then?:
It is WotC's policy to sell the PHB, DMG, and MM at a reduced margin in order to make the game accessible and attract players, postponing major profits until the supplements are released. This is done in a manner similar to the way that game console manufacturers often sell their consoles at or below cost in order to generate an installed base to which they can sell games at high margin, to make money on the game licenses.
The 3 core books are "loss leaders" that get you into the game and turn you into a possible purchaser of the supplement books. Someone without the 3 core books is not likely to be a potential customer, so it is vitally important to price the starter books in such a way that a new player will not feel like they're gambling a lot of money in order to get started with a game they aren't certain that they'll enjoy. This was the policy for 3rd edition books, and it remains the policy for 4th edition books, because WotC has determined that it is a productive strategy.
