The Gemara in Sanhedrin 108a at the bottom of the page says, the generation of the flood asked Noach, if the mabul is imminent, why hadnt it occured already, ie what was holding it back? Noach answered that Hashem is waiting to take away his bird.
Rashi explains (according to the way I understand it) that Hashem was waiting for Mesushelach to die, so that he should not perish in the flood with everyone else.
My question is why couldn't Hashem have brought the Mabul instantly, and simply included Mesushelach in the Teivah with Noach and his family?