The problem with small landlords is they can't average bad tenants over a large number of units. They have one or two units, and even one just sitting empty for a few months destroys years of cash flow.

Let alone actually getting a bad tenant. It may be a low chance, but if it comes up, you can be hosed out of the entire value of the house (usually you can still sell the land, but in the absolute worst cases where the property was turned into a meth factory, you may be out more than the entire value).