Buyers will compare the debt-to-income ratio of any inquiring purchaser (whether they are a renter or a current homeowner looking for a new home). When an existing homeowner plans to sell their home once they purchase the new one, the mortgages from both properties will be counted in their debt-to-income ratio. Renters don't have this additional debt and so are more likely to qualify for a new mortgage - making them look like a better option.