Think of it this way. All natures boost one stat by 10% and lower another by 10%. Now, remember SpillThePolteageist? Putting 252 HP is almost always better than maximizing both defenses. This is because HP essentially increases both Defense and Special Defense in allowing you to take more damage from both sides of attacks, which are Special and Physical. Now, that gets rid of the entire purpose of nature by increasing one stat and lowering on stat. By increasing HP, you are essentially getting a 2 for 1 offer. Hopefully this makes sense.
Hope I helped!
Note: Just thought of this. They also might have done this to keep Chansey/Blissey less of a wall. Blissey with 780 would not be fun.