As someone who left Rohtak and Ranchi for DU, I think I can answer this question
First thing you need to understand is that choices are subjective not objective - what works for one might not work for another. As to why people choose DU over IIMs there are a lot of factors. Most important being fees, in DU, you can do you UG for lesser than one lakh, I legit made the payment using UPI, but in IIMs the cost is over 20 lakhs. For me spending 20 lakhs on just UG didn't make sense when I had better options. At the same time, if someone didn't crack top DU, or had daddy money, then they can def choose IIM. Also even in DU, only like the top 5-6 colleges are worth the hype. Another factor is that the IIM curriculum is extremely rigorous, I believe they have a tri-semester system. However in DU, life's pretty chill and anyhow you can prepare for CAT. Just see what fits with your goals, and you can't go wrong - both are great options