1) Even though the rainy season starts from Sukkos and we therefore start praising Hashem for the rain and saying the words “Mashiv haruach umorid hagashem” in the Amidah, nevertheless we do not start asking “Vesein Tal Umatar Livrachah” (and give dew and rain for blessing) in the blessing of “Bareich Aleinu” until the seventh of Cheshvan and outside Eretz Yisrael one does not start until the 5th or 6th of December. (Shulchan Aruch O”Ch 114)
2) The reason we do not ask for rain immediately after Sukkos is since in the time of the temple those who went up to Yereushalayim for Sukkos were travelling back to their homes in this time-period and we do not want the rain to disturb them, we do not ask for rain until we can assume that they have all returned home, and Chazal said that this is on the seventh of Cheshvan. (ibid). Even after the temple was destroyed the custom was that Klal Yisrael went up to Yerushalayim for the festivals, so Chazal did not abolish this law. (Ran Taanis 2a)
