Isn't that because people aren't trapped inside sneezing on each other and breathing recirculating heated air?
That is certainly a factor for transmission but the recent thinking is that the flu virus stays alive longer in cooler climates. It literally hangs in the air due to favorable temps and low humidity. The warmth of the tropical climate kills the virus while the humidity prevents suspension in the air.
The virus has to stay alive long enough to transmit.
That is my rudimentary understanding outside of being an epidemiologist.