I think a problem is there are no personality's in it anymore, great men like Schumacher, Senna, who made the sport entertaining. Also it has got quite political recently and the antics of those in charge can't have done much for its reputation. It may also be the fact that it doesn't get the coverage of sports like football and it isn't as ingrained in some country's culture as other sports (americans - baseball/basketball, england - football, rugby). It can also be rather boring, I find the BTCC about 100x more exciting.
Manchester United 2008-09 Season - Trophies & Records