For cars etc they can't be made small or safe or cheap enough (yet). Fission (what we have today) comes with far too many safety issues for non-military transport. Fusion (what Fallout plays around with) isn't yet viable even at large scale.
Should be fine. My first Fallout was Fallout 3, having never played any of the others. Game didn't assume prior knowledge too much. I believe each game (and now this series) effectively "resets" itself - each game is broadly the same (survivor emerges from Vault, heads into the unknown, etc...