I spent time growing up in a couple of different 'intentional communities', both of which are dying a kind of slow death for similar reasons (as are many others that I am aware of).
These communities were founded by groups with particular sets of shared ideals. Shortly after being founded they started to welcome new members - offering a share in the assets of the community in return for labour and commitment. This generally worked well.
The problem is that as these people approach retirement they begin to realise that the community and its assets are all they have in the way of a retirement fund (sometimes also a modest private fund as well but their share of the land they live on is usually their most significant asset). This realisation is usually enough to stop or slow the practice of giving new members a share in the community's assets. This in turn makes joining the community less attractive.
On top of these financial concerns, there is the lack of shared ideology from one generation to another. The children that grew up in these communities prefer to go off and live their own life instead of carrying on with their parents dreams.
I am only aware of a couple of communes here in NZ that appear to be making this generational transition successfully (most communes here are approaching 50 years old, having been started in the 70s by people in their 20s).