"Danzō could utilise multiple nature transformations which included Wind, Earth, Water, and Fire Release, as well as Yin and Yang Release.[2] He was particularly proficient in Wind Release, exhaling powerful vacuum-based attacks that could either slash like blades or pierce like bullets."

Quote from Naruto Wiki.

My theory for why he is fire here- there was need for Kage-class ninja with Fire-Lightning barrier- and he actualy c*e all of the styles- and they also didn't want this version of him to be too op- so he doesn't have all of releases included in type of his attacks damage.

That's how I see it