its the same reason as why people want Naruto to become a chuunin

its nothing more then a rank in the village
Kabuto never got past genin in ranking, so he remains a genin - no matter his strength.
these ranks determine what missions they go on, thats all it does, Kabuto doesnt deal with missions in the same way as the leaf nin do so for his purpose he doesnt NEED to be a jounin or a chuunin