In the lore script i just found it states

"Diablo's essence was finally trapped in a soulstone and banished to the unfathomable abyss 20 years ago"

This is a lil wrong from what diablo 2 storyline says if you ask me...

Diablo's essence was only trapped in the soulstone (which they really turned into a delayed gateway from what the fallen angel said) after diablo 1

In diablo 2 you kill diablo when his soulstone is in his head, and then later destroy the soulstone.

And from diablo lore, i thought if a demon was killed (hell or earth) that they were just to be reborn in hell eventually, which is why diablo is back....

the soulstones being destroyed doesnt stop them from returning, they were a trap to bind them inside it, and they failed and proved to be more of a help to the demons then a trap, thats why they were destroyed.