Suppose you own a house and a ten unit apartment building. If you have a vacancy in each building what do you have? A 100% vacancy in the home and a 10% vacancy in the apartment. Vacancy means no income. Plus you would still have to pay the utilities. With the apartment, you would have the 9 other properties pulling money in.
Lenders would rather lend large amounts of money over smaller amounts. Naturally they want to lend more, because that means they would get a higher commission from it. Buying single family homes, the borrower must qualify for the loan. They look at the borrower first then the property. With apartments it is the opposite. Logically if you own a 25 unit apartment building and the debt service/mortgage is $15,000 a month very few individuals could support that payment on their own. The lenders want to know how much income the property brings in. The property is the business so to speak.