Here in Phoenix both ABC and NBC run their national news at 5:30 local whereas CBS is 1/2 hour behind starting at 6PM.
Do you think people who are actually interested in these glorified prescription medicine adverts are just unwilling to wait the extra half hour?
Not sure how many other major metros air the same schedule.
All three run at 6:30PM in Detroit. ABC, for years, ran at 7PM here but, they too eventually switched to 6:30. The CBS affiliate likely suffers here from the lack of a local newscast as a lead in.