There is not much you can do. You cannot force guests to leave a review and guests not leaving reviews happen to all of us from time to time. They get plenty of automatic reminders through airbnb.
It would help if there was a way of penalising guests who do not leave a review. Just putting down the number of stays and the number of reviews on their profile page might do the trick. So if they've stayed in 10 places and left only 1 review, you might not want to accept their booking.
Some guests do not leave a review in the hope that they will not get reviewd in return as they may not know exactly how the review system works. Some even avoid airbnb for the review of guest possibility/reason, they do not want their activity spread in public.