Because a bijuu is a bijuu. If PS was as strong as Kurama he would have said Kurama instead of making a vague statement.
According to Madara, one swing of its sword breaks all creation
Well Madara is a sad schmuck then, since Shinsusenju tanked 5 swings coupled with 5 bijuudamas, so let's stop wanking his words as if he is the prophet of our lord and savior.
Madara was never one to take something weaker than him along. He needed the Kyuubi to face Hashirama and he needed genjutsu to make it fight for him.
It is obvious that the Kyuubi added something extra to his arsenal.
And a simple chakra roar laughs at PS' shockwave. PS knocked down five men, out of which one is 73 years old and is one meter tall.
A 50% Kyuubi's chakra roar knocked down 5 bijuus.