The imprint of his personality left on the flower was brought back to life when injected with determination, but he no longer had a SOUL so he couldn't feel positive emotions. Apparently.
True Pacifist ending kind of implies that he at least wants everyone else to remain happy and begs you not to restart your game.
Because you just haven't hacked the game to add a journey into Asriel's mind with [Playername] the Fallen Child's Influence (played by the Fallen Child themself) as the true final boss, which would allow you to save Asriel. Same reason you can't also save
>actually using furfag words like murr and yiff unironically
If the goat child doesn't want to be saved, he should rot.