“Why a rebaptism for Catholics?”
Because some of the Orthodox no longer follow the Canons of the first 7 ecumenical councils with regards to this matter. ;) However, not all Eastern Orthodox follow this practice and some will even receive Catholic clerics in their orders!
However, the Oriental Orthodox are the worst anabaptists. For instance the Copts will rebaptize all Chalcedonian Christians - even the Eastern Orthodox! It is all quite scandalous really and certainly is proximate to heresy.
Really? Which ones?