It's definitely not as straightforward as simply not building enough houses. That quote from the article is dumb, they may as well say "failure of the past to predict increased demand on housing in the future has caused a shortage of housing in the present". A pointless observation...

Not to say there doesn't seem to be an issue brewing. Maine (and much of the US) has been pushing unsustainable growth since the late 40s, and the downsides of that is becoming more apparent.

Development in Maine has trended towards sprawl. A few relatively dense population centers surrounded by networks of suburban sprawl. Portland, Brunswick/Bath, Lewiston/Auburn, Augusta, Bangor...

In addition to increased demand from people moving to Maine from out of state, demographic shifts within the state are placing pressure on the denser parts of Maine. People who 30 years ago built 30+ minutes out of town using gov loan programs and cheaper land, are now realizing they can't afford the upkeep and distance from services as they age. So they sell or refi and buy something in the dense part of town. Problem is even younger folk on Maine wages can't afford to live 30+ minutes from town with rising prices, not to mention deferred maintenance on the homes.