People like to see their heroes fall. It's kind of strange, but that's human nature. We build them up, and we love to tear them down. This is true of sports figures maybe even more so than other celebrities, because many people resent the amount of fame they get just for playing a game.

There are a number of reasons for this.  The two most important are:

First, Simpson was a very big name in the world of American football.  He was one of the greatest football players ever, had been a broadcaster, and was very well-known to American sports fans.  Celebrities always make for big news and he was a celebrity.

Second, there was the element of race.  Simpson is black and his wife was white.  There were allegations of racism among the police.  These things focused attention on the trial and it turned the trial into something of a racial issue.  When the verdict was handed down, the reaction to it was almost completely along racial lines.

So, in the Simpson case, you had the intersection of two major US obessions -- race and celebrity -- to go along with gory murders.  This is why it was the "trial of the century" -- not because it was particularly legally important.