Why no discount for long time customers

I had to replace my son's LG Charge today because it looks like the battery is swelling up and pushing the case out. I got the LG K51 for him since is was in our price range. While it offers a $100 discount, it seems to only apply to new lines. Why cannot long time customers get a discount since the phone is much cheaper if I could buy it dirrect from LG.

Pat Geary