To echo what others have said, this has everything to do with price point and length of cruise, which dictates what kind of clientele will be attracted.
In the past few years in particular, you can/could get dirt cheap direct flights from NYC, NJ, or Philly down to Miami. Combine this with the 3-day Carnival cruises out of Miami generally being the cheapest kind of cruise there is. You can do them in a weekend. So they cost relatively little time and little money, thus they are going to attract a party-oriented crowd. So it's not Miami per se that's a problem. It's just that for those of us in the Northeast, it's a short, cheap, and direct flight, as well as a cheap cruise. Win-win for many young folks who don't have a lot of money, but are looking to party and sometimes let things get out of hand.
If that's not your thing, then I suggest not going on short cruises. I went on a 6-day Carnival Caribbean cruise out of Miami a few years ago. It was more crowded than my liking, and there were a few people who got too drunk and became unruly, but in general, I didn't have any complaints.