Reanimation of Yondaime during Chunin exams
Remember the Orochimaru did not know of the Shiki Fuujin, he didn't know about not being able to summon the fourth. The Shiki Fuujin was kept highly confidential and was only ever used twice throughout the series.

Now for a more practical explanation, the author likely didn't think that far ahead, and caused The Third to be able to "cancel" the third summon. The Shiki Fuujin explanation was later applied an implied by certain events taking place.