Two words: Local News.
For local affiliates, local news is a major moneymaker. ABC stations in the Eastern and Central time zones would have been irate at the network if their Sunday early-evening local news was pre-empted by a late-running game.
Perhaps ABC was also worried that if the telecast ran to 6:30 P.M. EDT, that the network's evening newscast would not air in most of the Eastern and Central time zones since prime-time began at 7 P.M. EDT, and had the Little League World Series telecast ended at 6:30, ABC's affiliates in the Eastern and Central time zones would have had local news, but no network news.
Normally, ABC probably could have dumped their network news in the East and Midwest. But Hurricane Katrina was approaching Louisiana, and the network may have wanted to have had at least an abbreviated network evening newscast.
Actually, I think ABC should have stayed with the game and should have given their local stations (in the Eastern and Central time zones) 30 minutes of local news after the game coverage ended. If the end of that local news was after 7 P.M. EDT, ABC should have pre-empted whatever was scheduled for 7 P.M. ET and run an expanded "World News Sunday" up until 8 P.M. EDT, with plenty of storm coverage.