Before we continue, Kekkai Genkai is a gifted ability to combine any 2 elements.
She is my favorite and one of the few people who can use 3 Elements. Water, Fire and Earth.
According to Yamato, Mokuton (Wood Release) is a Kekkai Genkai consisting of Water and Earth. People who has access to Mokuton, users is shown to have access to Earth and Water style.
Now why can't Mei use Mokuton? She is able to combine Water and Fire to make Steam or Mist, and she is able to combine Earth and Fire to make Lava.
Why can't she combine Earth and Water to make Wood Style.