Because there is no time when he ever asks Sarah to clarify it, waits for her answer, and then translates her answer.
That does not accord with what I hear on the tape. At 4:58, McRae finishes asking if Obama was born in Mombasa. At 5:04, the first translator starts speaking. It then becomes hard for me (not speaking either Swahili or Luo) to figure out when the first translator stops and the second starts. At 5:20 I can definitely hear a female voice, but it is very faint, as it is whenever the female voice speaks. At 5:25, (21 seconds) the translator speaks English "no, Obama wasn't born in Mombasa, he was born in America."
Listening to the earlier portions of the tape, there were several exchanges. One exchange had the McRae portion finishing at 2:38, and the English translator answer beginning at 2:54 (16 seconds) and another had Mcrae finishing at 3:36 and the English answer starting at 3:51 (15 seconds). So I'm not sure how you figure there was insufficient time, since the earlier exchanges were even shorter.