Because these are the senior notes of the holding company, they have a higher position in the capital structure than the common stock - meaning that Primus Financial Products would have to suffer more extreme losses to its swap portfolio before the senior notes are impacted, relative to the common stock.
From http://collegeanalysts.com/2008/06/26/last-comments-on-primus-prs-for-a-while-i-promise/
Disclosure: I own PRD.