We do wait for travelers before praying for rain. The reason given (Mishna in Taanis 10A) for why we wait 15 days after Sukkot, until the 7th of Cheshvan, is in order to ensure that the last travelers can pass the Euphrates River before the rain comes (Rabbeinu Gershom) and makes the river difficult to pass (Rashi).
Once the 7th of Cheshvan comes, all the travelers have left the land of Israel, and Israel can start praying for rain.
The Gemara then continues to tell us that outside of Israel they wait until 60 days after the Tekufat Tishre to start the prayer for rain. Rashi says that this is because Bavel is a low place and therefore doesn't need as much water. Rashi later says that we (the rest of Chutz LeAretz) follow the customs of Bavel. The Mishna Berurah (117:1:5) says the same thing.
Shulchan Aruch HaRav 117:1 brings an alternate opinion brought by the Rosh. This opinion questions this and says that while it is true that we follow the customs of Bavel, this is only on matters of Halacha, not when we need rain. Every region should therefore start praying for rain whenever they need it.
This opinion was not accepted by anyone, even the members of the Rosh's synagogue (see @DoubleAA's comments). See there for more details.
If a small area needs rain before the 60th day of Tekufat Tishre, they should add the request in the silent Amidah during the Shomeah Tefillah blessing.