Venus won because she extended long rallies & adapted to the wind
Serena came out swinging and hitting winners from the first shot. Serena always wants to hit quick winners and aces and end the points quickly. And she did that for the first few games.
Venus just kept her cool and settled down and played herself into the match. Venus began returning deep and started the long rallies which wore Serena down mentally and physically. Venus adapted to the wind and was patient and she kept catching her ball toss when the windy conditions were not right. Serena wanted to just get the points over quickly did not adjust to the wind.
And it actually was the second serves that won the match. We know the two sisters will win points on their first serves and aces. But on second serve, Serena kept hitting that high kick serve and it sat up for Venus to pounce on it.
Where as Venus kept hitting the second serves fast with a lot of spin and it was spinning away from Serena and she couldn't handle it.
Then those long matches were Venus was running her around the court and that did Serena in because she had no more strength to hit the hard serves.
Venus strategically kept getting those balls in play knowing Serena likes to end the points quick and then follow up with an ace.
Venus played a grass court game with power and athleticism.