Why is the P-521 elliptic curve not in Suite B if AES-256 is?
The real question isn't "Why doesn't Suite B use P-521?" It is, "Why doesn't Suite B use AES-192?" NSA were only interested in 192-bit security for Suite B, but they chose to use AES-256 because AES-192 wasn't widely supported.


  "In fact we had wanted to use AES -128 and AES-192, but a quick survey of AES implementations (hardware centric, I believe) showed that there were very few implemented AES-192, whence the decision to go with AES-128 and AES-256 in Suite B, paired with P256 and P384.   All of the crypt purists grumbled endlessly about the mismatch betwixt AES-256 and P384."
  
  -- Kevin M. Igoe, NSA


(I am choosing to take his explanation at face value, but you never know with intelligence agencies.)