Because as opposed to what Moffat would want you to think, he doesn't plan ahead. He makes stuff as he goes along, so he's bound to not remember the billions of lose threads he leaves around.
As far as I understand it, Eleven was going to be the last doctor, meaning no more regenerations because he has no more regeneration energy. When River killed him the first time in "Let's Kill Hitler" he was really dead, but River used up all of her regenerations to bring him back to life, giving the Doctor some regeneration energy. I'm led to believe that the amount of regeneration energy he received was just enough for one more regeneration, and any other energy was used up just by bringing him back. Now we can say that the Doctor gave away some of that energy back to River to heal her before he began regenerating the second time when he went to Utah, which could also explain River's worrying about the Doctor using his energy when she knows he has very little left.
Of course my theory could be totally wrong, and it could just be >gif related.
Yep, that's why he was able to regenerate into 12 (or 13 or 14 or however way you want to count), but that was after all of this when he ran out energy again, and he would have truly died on Trenzalore if the Time Lords didn't give him more energy.
He's able to use small bits at a time. Nothing near fully regeneration, but enough for a broken wrist. Remember that a lot of energy is discharged and in excess during regenerations
I just enjoy living in my world where when River gave the Doctor her regenerative powers to revive him she also gave him the rest of her regenerations. Quite a few flaws with this due to the Christmas special, however it lets me believe that there could be many more generations to come. Let's just face it though.....Time Lords will come back, **** will go down, Time lords will give the doctor another 13/infinite lives, we will spot problems, and finally we will all just enjoy the following doctor