Sports bikes are for pushing the limits, cruisers are for laid-back, low-pressure riding. Pushing the limits appeals more to young testosterone-poisoned men who think they're immortal. Cruisers are more for guys who just want to ride and enjoy themselves and not pretend every ride is a Grand Prix race.
Also the seating position on a real sportbike is extreme--all leaned forward, stretched out like that--while cruisers have a more upright posture, or even leaning slightly backwards. Older people have trouble with the sporting seating position. Plus the seats on serious sportbikes are small and hard, because you get a lot of feedback through your butt. Cruisers have big, cushy seats that you can ride in all day.
If you have a few extra pounds on you, if you're a guy, it's usually in a 'spare tire' around your middle. Guys with a pot on them look really funny on a sport bike, with their big gut squished on the tank. On a cruiser they look just right.
There is a middle ground! But generally sport bikes are for the performance-oriented and cruisers are for the comfort-oriented, and BOTH styles appeal to the style-oriented.
