Okay.

Possible Answers:
- The climate. The south was known for being much warmer and Cao Cao's men suffered from sickness coming down from the north.
- Cao Zhi's poem, which threatened to take the two Qiaos to the north and install them in the Bronze Bird Tower Cao Cao had built for them. This angered Zhou Yu.
- Cao Cao's men were mostly from the north and unaccustomed to naval warfare. Zhou Yu, however, was a master of naval strategy.
- Cao Cao, upon deciding to attack the south, was taking a huge risk leaving Ma Teng unchecked to the west.
- Zhou Yu had made a promise to his former lord, Sun Ce, that he would never submit the south to Cao Cao. Sun Ce had, on his deathbed, insisted that all external matters should be referred to Zhou Yu. To just simply surrender would have gone against that dying wish.

I'll ask an easier question:

How did Zhou Yu die?