There are a number of reasons why many top rankers in JEE Main choose DTU over IITs and NITs. Here are some possible reasons.
In JEE Advanced if your rank is 2000+, your options for the branches are limited in IITs. Those who have a specific goal and preference then prefer going for a tier-2 institute that lets them. Some want the IIT tag and compromise on their branch.
It’s the ones that look at the bigger picture and compare colleges on wider aspects than the brand value that end up choosing DTU, the best choice after IITs and NITs.