here are a few:
generally good tempered, allow handling better than most snakes, captive conditions are well known by many people because they are common, stay fairly small (for pythons anyway), BEAUTIFUL, plus they have many other good characteristics that other snakes have like not requiring extensive amounts of time to care for (feed infrequently, and dook infrequently), and the list goes on and on!
If you tell us specifically what you want this info for (who you are trying to convice or whatever) I'm sure we can come up with some good reasons for you.
Ben