Recovery due to an in-flight abort would be difficult in rough seas and weather if they had to be plucked out of the Atlantic.

The aerostructure of the falcon 9 can only withstand so much side loading due to aerodynamic forces and wind before exceeding the operational limits of the rocket (body to keep it all from breaking apart, and the engines working hard to keep them on the correct trajectory).

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.

Edit: and for the falcon 9 (non crew) https://www.nasa.gov/pdf/649911main_051612_falcon9_weather_c...

Given what the weather has been like since Sunday I could see it being postponed. I hope the weather holds and gives a window.

It misses the upper level winds, and for the human launch I wouldn't be at all surprised to find out that it is missing some weather criteria in the abort recovery zone as well.