User blog comment:ImperiexSeed/Supernatural Mysteries/@comment-1970092-20130106012304/@comment-3481504-20130106202329

Well, Castiel said he wanted Hell active as a "threat to hold over [his] enemies." Before that, he said, "[I'd] done away with it COMPLETELY." And Lucifer's cage is actually in Hell, likely within the Eight circle. So if he destroyed it, he would've either eviscerated everything occupying it, or Michael would've survived, and crawled out.