One of them is caused by geographic separation. Without either proximity or regular public transit to a nearby population center, you lose your ability to market the community to potential new members. Like Apple, or Tesla, in order to break into an entrenched market you need visibility in high foot traffic locations. Regular festivals that draw crowds, retreats, etc, can help.

The other big one is unity of vision. Many of these communities lay down very specific rules in their bylaws. But humans tend to go through cycles in their ability to stomach group strictures, no matter how noble their intentions. Without an outlet on the property for counterculture, residents are pushed out every time you go through one of these cycles. Often these strictures come from a central personality. See Arcosanti and Paulo Soleri's refusal to entertain young anarchists vision for the place.

I would also say many of these communities fail because they don't separate enough from mainstream governance concepts. Many require you to make a big financial investment up front, or use a democratic governance model. Neither of those should be necessary in the age of microfinance and cryptogovernance, but we're only human, and it's hard for us to give up old habits.

And lastly, poor generational diversity is huge. Most of these communities die of old age, with a few geriatrics keeping the dream alive while their secular children live as close as they can, but outside the "community". You must have immigrants in every single age bracket throughout the life of the collective. As soon as you miss a generation you make it exponentially harder to recruit for the following one.

I would be interested to see a city concept based on freedom. Big enough for good separation between activity centers, and designed to support bifurcation of governance as much as unity. "What if we didn't have to agree?" would be the motto. Not "should you be allowed to do that?" but "if one were to do that, where would the best place for it be?"