I think it's a combination of xenophobia (am I crazy, or was the Andalite culture partially inspired by Japanese culture?) and the fact that the consequences were SO HUGE, and I get the idea that the Andalites weren't much for cultural and technological exchange before then. IIRC, we've only seen them work with other aliens when they were defending the aliens' home planet from the Yeerks -- i.e. it was a case of the stakes being high enough that they would overcome their disinclination to interact with other species.