The scriptwriters or Deng Chao or both seem determined to present a different portrayal of Wuji.
To go off on a tangent: Alec Su's Wuji in comparison spends relatively longer time being nice to Alyssa Chia's Zhao Min, and the chemistry between them shows more often, to the point that Alec's Wuji sometimes acts a little out of character. For example, in the dungeon at Green Willow Manor, when Zhao Min asks, "Why do you keep holding my hand? We're a man and a woman!" (before the foot-tickling) Wuji's hand lingers on hers before releasing it, while other Wuji's usually snatch their hands away at this scene.
Thus, 2009's scriptwriters may want to underline the fact that Wuji and Zhao Min start off as foes and don't fall in love until much later. I find Deng Chao's Wuji to be a bit more sadistic (compared to previous Wuji's) in the dungeon scene, only showing his remorse and compassion when Zhao Min starts crying in earnest. That's like going to the other end of the spectrum, after Alec.
Another maybe: Deng Chao's Wuji may come across as having little chemistry with the girls in general because he's so confident, often fierce, and shows Wuji's soft-heartedness less visibly. While in previous versions, Wuji's apparent soft-heartedness makes him appear more empathic and attentive toward the girls.