Let me explain it to you.
There must be a beginning in everything. A kkg is a blood ability but also it could be the ability to learn it.
For example Black lighting and dust release are consider kkg. Mu show Onoki how to use it, but Onoki needed the ability to use 3 element and combine it. Sometime might be a clan's secret tecnic which is past down to future generatios. Ei 3th taught Darui the tecnic. Kkg combines elements to create something new. Haku must have learned it to. Also it might also be related to blood line, for example the Sarutobi clan, they might not have a kkg but all the Sarutobi tend to have fire release as there prior affinity, so then maybe the Yuki Clan where Haku is from tend to have water and wind release as natural just like the Sarutobi's fire release, which makes it easier to learn ice release with hard train.
Kkg like sharingan and byakugan are blood line kkg.
Yamato even said he has to combine water and earth to make wood buy naturally he was gifted with that talent to combine it with ease. Conclusion, there can be more wood style users but it is very hard to learn and have the requirements.