A small question. 3D is not in the Fallout 1 and 2 games. Why are we adding it? It's going to start feeling not at all like the good old classic games.
A small answer. Because 3D is much easy to customize. Original fallout engine is not suitable for MMORPG, it is completely exhausted already. Adding new critters is a hell long laborious process. Adding new animations for all critters is nearly impossible.
3D is flexible. You can add new critter and it will take all present animations automatically. Or you can add new animation and all critters take it. You can add new item and attach it to any critter. If you will read this branch of forum more closely, you'll find different playable critters (instead of only vaultsuit), different skin tones, tattoos, casual suits, etc. Adding it to an original 2D engine is impossible. 2D is not a feature, it is just a technical restriction.
Have I answered your question?
P.S. and a little correction: you do not adding 3D, we do it.