Because it is a well-built vehicle, rather simple (inline-4, DOHC engine, front-drive, four-door sedan) in design & purpose and it doesn't cost an arm and a leg to own, drive or maintain.
Notice how other manufacturers have tried to "copy" the concept. Look at a Honda Civic (another great car), the Nissan Sentra and even the Hyundai Elantra -- three other decent cars that each have their own following.
~ "If I passed you on the right, you're in the wrong damn lane!" ~