Why can you not package lentivirus using a retroviral packaging system and vice versa?
There are many reasons why not, most of them at the molecular level. For instance the packaging signal (psi, located on the lentiviral vector RNA) which facilitates incorporation into the viral particle through an interaction with the gag polyprotein, is not recognized by the gag polyprotein of the retroviral packaging system. However, it's more complicated than that, because just simply switching the packaging signals doesn't work either. Many researchers have tried unsuccessfully to get this to work.