You'll have to ask Ranieri and Secco.
My guess is they are either happy with Zanetti with Tacchinardi/Blasi as backups, or they are opting for a cheaper DM solution (Mavuba)... coupled with the desire to spend the majority of money on one major midfield signing like Diego or Lampard. Not to mention, Ranieri likes youth, so odds are Marchisio, Nocerino, Paro and Giovinco are a significant part of the midfield equation.
Of course, this is all conjecture.