There are a couple of issues raised in the question. On one hand, I think that the ghazal's lasting popularity represents how the power of tradition is still a part of Indian identity. The ghazal's history stretches back generations. At a time when Indian cosmopolitanism as well as India's active participation in a globalized world might do damage to some aspects of its traditional culture, the ghazal stands as an element that thrives despite its age. From an artistic point of view, the ghazal represents the very essence of classical training of the singer's voice. Part of the reason why few singers are able to embrace the form is because the ghazal requires a classical sensibility that demands the highest of training. Only the most talented of singers could attempt to sing the ghazal. Artists like Mohammed Rafi, Hariharan, and Pankaj Udas are few of the crossover artists who could sing popular music while adhering to the strict standards of the ghazal. Nusrat Fateh Ali Khan and, more recently, Rahat Ali Khan are also examples of the artists who could span both domains.