1) Everybody watches the super bowl, not just hardcore younger fans. Frankly, I couldn't tell you the last time I watched more than 30 seconds of the halftime show, so it matters not to me whether they put on the Who, Jessica Simpson, or an Elvis impersonator. I guess they're figuring that The Who and Tom Petty are at least palatable to most people, even those who aren't fans.
2) Many current artists in pop music (including hip hop) are simply not trustworthy to put on that stage. Just listen to their lyrics. And it's not just hip hop. Artists of other genres who would seem to fit aren't trusted either. Mike & Mike had a long discussion about this one day. They mentioned Springsteen and how he would appear to be ideal, but he's so political, they probably wouldn't ever put him on. There are obviously some they could probably trust, but many of them are too much of a threat to pull something like the Janet Jackson fiasco or spout a few f-words.