Simply because Hashirama is a boss....hell idk, odds are Hashirama had more then just wood release seeing as he bumped heads with Madara time and time again and managed to be considered equal.
I have a theory of my own...Yamato's wood release is an extremely diluted version of Hashirama's and seeing as Yamato's wood release had abilities in supressing the kyuubi's chakra, I believe that the actual kekkei genkai holds stronger versions that we haven't seen yet. Just as Sharingan has it's upgrades, Wood release should have it's own levels as well. This is my theory, Wood release and Hashi's abilities just haven't been explored as much, in terms of fighting and battle potential. Lets see how Zetsu is turning out to see if we can make some sort of justifications on potential abilities of Hashirama.