Why do live broadcasts in Dolby digital sound like crap?

I have a 5.1 setup that sounds great with streaming movies etc. but whenever I watch something live like the Super Bowl or the Grammy’s, the sound is very lack luster. Last nights halftime performance had no bass, yet my receiver was receiving a Dolby Digital 5.1 signal? The music level was low and the vocals were louder than they should be. Why?