Why would that be the case? These places are currently not being rented at all. Why would people favour short-term arrangements? I own 2 apartments and rent one of them (mostly to students). I look for tenants who can stay for longer periods so I don't have to spend a lot of my time finding new people.
"I'll lower the rent until I can find someone to pay more" is the very definition of free market; a free market in which rent rates stabilize downwards, as socially desirable.
Attempting to rent the house for a shorter time is simply going to restrict the target market to transient residents who plan to move away shortly (e.g. students), which is rather orthogonal to being rich or poor.
Owners need to seek a balance between not renting at all, or renting very cheaply, and accepting longer-term contracts; it's never a unilateral decision.
