Obviously, I can only theorize. Conservative theory: The clan that built the Unown temple wanted the Unown to draw powers from ancient Pokemon in order to teleport to the Pokemon world. Ambitious theory: The clan wanted the Unown to create those Pokemon, which had only been a figment of the imagination. Perhaps the Unown also created Mew, and in turn, every Pokemon. Bear in mind that the second theory isn't supposed to be consistent with anything but the first two generations.