I haven't looked too in-depth at it but hasn't it been said players are hitting their primes younger (around age 23?) so maybe he is simply exiting his prime. From ages 19-23 he was a PPG or higher player, age 24 he was 10 points below a PPG and last season 13. Also for a good part of his high scoring years the two main guys were St. Louis and Stamkos and then whoever got to ride shot gun on his line.
The last couple of years the Lightning seem to be more balanced, with guys like Kucherov, Palat, Johnson, Filppula, Callahan, Drouin, Namestnikov, Killorn all cutting into the production of the higher end guys. (Some of them are higher end mind you).
With potentially 9 guys who have showed ability to score in this league, the point totals of Stamkos are going to drop because there is only so many goals to go around. The better the team, the less points you're going to get individually because no matter what team you are, you likely aren't going to score over 3 goals per game, so same amount of goals and more players to share those goals with.