1. Those bad tenants made troubles even with good credit score or
2. They don't have good credits or
3. The landlord didn't check for the credits.
I personally wouldn't rent to people with bad credits for I have a perfect 815 credit score myself. :) (Can anyone have 900 full score?) And I personally wouldn't rent to certain kinds of people. My houses are far below 1:100 but good night sleeps are much more important for me (this is why I don't invest in stocks, commodity, etc.). I guess my principles wouldn't work for professional landlords. But if you only rent out houses to keep your extra cash above inflation, it works fine.