The problem started long time ago when the central government took most of the tax revenue from the local provincial and city governments leaving them without funding to operate. As a compensation, they were granted the authority to sell the land use right of their lands. Land is never privately owned in China. What is sold is the right to use the land for a period of time.

Since the state owned all the lands and the local governments as the sole supplier of lands, they became a monopoly in land sale. As with any monopoly especially a unchecked one they pushed to raise prices all the times to gain maximum profit. The regulation and policies of the governments aimed to build more and to keep raising the property price years after years because that's what brought in the money.

It has become unsustainable as people couldn't afford the high price. There're a glut of houses and sales have been slow yet the governments have prohibited the lowering of the price as lower housing price means lower tax revenue. There're some cases when developers lowered the price and got hit with huge fine and jailed.

High price and slow sales mean the developers like Evergrande cannot get rid of the inventory to get the capital back to fund further development. They have already sold the not-yet-built houses for the next phase but have run out of money. Couple years back the central government had forced the banks to put in severe limits on loans to the developers. They turned to shadow banks like investment fund trusts to raise money but those are blowing up just now, defaulting on bond payments. They have no money to finish the projects. They owe huge amount of debts, owe huge amount of unpaid taxes, and owe the unbuilt houses to customers. They should have been bankrupted a long time ago but the governments won't let them.

This is a huge problem for China. Housing development comprises about 25%~30% of GDP; its blowup would cripple the economy. Most ordinary people's wealth are largely tied to the value of their homes; a large drop of the house value hits most people hard, forcing them to cut back on spending, further putting down the economy. The financial institutes like banks, shadow banks, investment funds, etc have lost huge amount of money as developers unable to pay back their loans and people defaulting on their mortgages. The lowering of the property value would force more loans gone underwater. There'll be a string of bankruptcy. The local governments have large revenue shortfall and yet saddled with huge debts, because guess what, they were in the game to invest to build housing. The shortfall forces them to lay off workers, cut back on salaries, cut pensions, and cut back on services. Even the central government has no money. Last year they openly said that beyond some emergency fund there's no money left.

Any option for solutions is a bitter pill and no one has the political will to do it. It's just a downward spiral.