Because of Their Support

The surge of vines tumbled along the ground, pulling up waves of earth as it did.

What surprised people was that the vines weren't going toward Ashe, but instead, toward Thresh who had been about to launch his scythe. 

Side step and avoid Thresh's scythe.

Luocheng was confident in his ability to side step because for these kind of champions that hooked, he was able to accurately predict where it would come from and where it would go!

Just as Ashe moved back and forth to juke Luocheng's skill shots, who knew Luocheng would actually turn and launch his Grasping Roots toward Thresh instead, completely ignoring Ashe.

This completely threw Ashe and Thresh off.

The vines shuffled over Thresh's feet, wrapping the lower part of his body and rooting him in place. 

A poisonous flower emerged like a sharp tongue from the earth, spitting at Thresh which applied a slow effect on him.