If you're changing your models names constantly, how will the public remember what it is that you're selling?
Good example, as mentioned already, is the Jetta. It's VW's best-selling car in the US so there's value in keeping the same name that buyers recognize. Yet in Europe, the Jetta has always been less popular than the Golf. Thus, in Europe, The Jetta became the Vento, then the Bora and then the Jetta name was brought back.
Sometimes I scratch my head as to how GM once abandoned names like Impala and Monte Carlo in favor of names like Lumina.