With enough money, you can afford to wait for a good deal because you don't need the money right now to live. When you're wealthy, you can brave health issues, accidents, sudden crossings with the law, etc. without ever truly touching the money-in-wait for a good deal. When you're middle-class you definitely don't have as much cushion.

This is similar to the logic of "the best time to buy is a recession", except its difficult to buy during a recession purely because it's more likely that a middle class person's cushion is gone due to economic conditions and thus has no money to buy good deals.