Generally they would be older airframes where winglets weren't an option at the time of manufacture. Also, if you are running a lot of short flights, fuel cost

Generally they would be older airframes where winglets weren't an option at the time of manufacture. Also, if you are running a lot of short flights, fuel costs may not be your primary concern. For example, the 747-400D is a model meant for short domestic routes like between Japanese islands, and doesn't have winglets.