Because it will defend against pretty much anything? 4 combined attacks of High tier shinobies (Gaara and co vs Sasuke) didnt do anything to a mere imcomplete Susano.

Susano had yet to fully materialise

Susano had yet to equip it's armor

And Susano wasnt in it's perfect form either.

And thats without Yata's mirror.

The only way you can deal significient damage to it is by attacking from a point where he can,t defend itself (Danzo vs Sasuke)... and even then the damage is enough to get through to hit the user.