First of all, weather is much more than just rain. One of the things that's monitored closely in the launch corridor are the upper atmospheric winds. Specifica

First of all, weather is much more than just rain. One of the things that's monitored closely in the launch corridor are the upper atmospheric winds. Specifically the wind sheer in the upper atmosphere. Strong wind isn't typically a problem, but high wind sheer in the upper atmosphere can be problematic. Additionally, they check the weather in the entire recovery corridor. That is, anywhere that the capsule could end up during an abort. They want to make sure that if the capsule ends up in the ocean, the weather permits recovery operations. An abort and safe splash-down doesn't do us much good if the astronauts then drown because recovery operations are impaired due to weather. We're putting two lives on this rocket. A small delay is absolutely the right trade-off to reduce risk.