This was decided before Katrina. The initial proposal by Congress was in 1996 to swithover in 2006.
It's motivated by a number of factors.
Bandwidth for emergency services but also commercial uses as a good chunk of the bandwidth was bought by Cell phone companies.
More programming on less bandwidth, due to compression. Higher quality programming (theoretical--depends on compression and other format choices)
More control over the media you watch. Not your control, but the broadcaster and producer. You might look into the whole broadcast flag
issue for some of those details, but also look at High-bandwidth Digital Content Protection - Wikipedia, the free encyclopedia
as well as making possible the expiration of content you record.
So it's mostly about money really. How to get more of your money in their pocket.
If you're on a cable or satellite system, you'll get grandfathered in. Not so if you receive TV over the air.
Most of the world is switching and some have completed their switch. Digital switchover - Wikipedia, the free encyclopedia