They may not become equal at any point of time as the level of students IITs possess is no doubt better than NITs.

NITs also have very good students but that is always the second choice of students in India.One would like to go to an NIT if he didn't get a good IIT.

Also, the level of studies in IITs is far more wider and higher level as compared to an NIT. The network of teaching assistants in IITs is better than an NIT.

Amount of research work, IIT's have good amount of research work being carried out jointly by professors and students. They do projects for esteemed institution like ISRO, DRDO, BARC etc. Even NIT's do research with them but not at the same magnitude.