mwh, maybe there is some explanation buried in the deep structure of Latin, but even linguists who study these things, after about six decades of trying to answer questions like this, don't agree on the underlying logic of language structures, if indeed there is such a thing. Why are some languages ergative? Why does English exhibit gender only in third-person pronouns? I firmly believe that the use of the imperfect/pluperfect subjunctive in Latin contrary-to-fact conditions fall into the same category of questions. Why does Greek use imperfect/aorist indicative with a particle in the apodosis for the same type of conditions? Russian also uses the past tense with a particle in the apodosis, but doesn't distinguish between present and past contrary-to-fact conditions. Why? I don't have an answer for any of these questions; do you?