The story is complex, but my understanding is that the main failure point was the local officials not wanting Arcosanti to exist and putting up regulatory barriers.
It's worth noting that Soleri ran the project as a benevolent dictator, and when he fell into the poor health of old age most decisions stopped. He's recently past and left behind a bit of a power vacuum and the other people involved are reorganizing to deal with that. Coming out the other side of that reorg is an opportunity for change and it's too early to assume Arcosanti will stay in it's current form.
They are basically blocked by money. If they could get enough money to pave the road to the property they could get building permits again. They need either a rich benefactor or a group of people interested in buying into the concept (and the leadership capacity to manage that).