A renter's financial risk is limited to his/her lease agreement. This could be the remainder of the rent due according to the lease (ie if there are 6 months left on the lease agreement you will need to pay 6X rent). In practice typically a renter is usually on the hook for the security deposit or a month's rent, so long as they can find another renter to take over their lease.

The owner assumes the total risk for the property.

As a basic example, consider starting a lease vs buying a property in 2007. If you were a renter when the real estate market collapsed in 2008 you were stuck paying higher than the market rate (most likely) until your lease ended, at which point your rent probably dropped to market rate. No biggie.

If you bought property at the top of the market in 2007 for say $500k, and the value of that property dropped to $250k in 2008, you are now paying a $500k mortgage payment for a property worth half that amount. Rent used to cover your mortgage payment but now the market rate for rent in the same property has halved so now instead of breaking even every month on your property you have to pay $1,000 a month for someone else to live in your property just to not default on the mortgage. Odds of having to declare bankruptcy in this scenario are high.