the fact that lillee happens to be one of the handful of modern bowlers with a match average of 5 wickets or more must be working in his favor somewhere. also, matches in england and australia would get better coverage in his playing days. better writers on cricket hailed from those countries too. both these factors combing with the genuine awesomeness of lillee created the aura around him in the cricketing world during his playing career.
as i had said before the critics look more favorably at performances in eng, aus and west indies than in pak, sl or nz. so lillee did well in england and australia. the matches were widely covered. fantastic articles were written on him. critics called him the best ever. rest of the pacers, like hadlee and imran and marshall, took inspiration from him. he became a legend.
but, later, when that same trio got bucket loads of wickets in nz, pak and india on leading their teams to famous wins. not everyone paid them the same kind of attention. in fact, they went beyond lillee in achieving more allround success in all sorts of conditions. even after that, in my books, all four are just about the same. to answer the question, lillee is rated above imran because he played in an era when performances in england and australia got more attention. it is a bit like haq playing in this tendulkar era when india is hogging the limelight and hence sachin is called god and inzy remains a good man at best. future generations might wonder why this happened so just like we are asking why lillee was bigger than imran.