Yes, there are number of students who had not joined IIT even after qualifying JEE Advanced . There are numerous reasons behind this

1- Some students give priority to the branch rather than to the college, so if they didn’t manage to get their dream branch they join other institutes which offers them their dream branch, these may be namely BITS, DTU and some private colleges .

2 - Some students crack entrances for foreign universities like MIT that are way above IITs, so they go for foreign universities rather than IITs, this has already happened thrice IIT-JEE 2006 AIR 1 Raghu mahajan,JEE Advanced 2014 AIR 1 Chitrang murudia and JEE Advanced 2015 AIR 1 Satvat Jagwani left IIT before completing their Btech and joined MIT

3 - There are many students who are interested in research rather than in engineering so they go for the institutes like IISc to pursue their passion and serve the country in a better way .