Why isn't Manafort turning States Evidence?

Because if he did he would be killed by Putin or the modern day version of the KGB now called the FSB.

This is a really simple answer. So, if you were in Manafort's shoes likely you would be trying to plead your innocence because it is the ONLY choice you have if you want to stay alive.

Basically, Manafort knows way to much to be allowed by Putin to stay alive if he starts talking about any of this.

I think it is important for people to understand this. In some ways the same is true for Trump. If he told the truth to the American people he might not be killed but members of his immediate family would be by Putin. This appears to be the case for both Manafort and his family and for Trump and his family.

This is my point of view as an intuitive.