The problem with putting the high quality subreddits on the front page is that most high quality subreddits can afford to be that way because of a relatively small and insular community. There is a fear frequently expressed on some of these subreddits that the quality will drop if they're put on the front page. Good examples are /r/truereddit and /r/moderatepolitics. Those subreddits have a comparatively higher signal to noise ratio due in part to their comparative obscurity.