Well to answer your question, let me ask you another.
Take a hypothetical scenario:
Say, you enrolled into a B.Tech course at an IIT and after four arduous years, you completed your course. Now instead of handing you a Bachelors’ degree, your college hands you a Bachelor of Science degree. How would you feel?
I respect those people who voluntarily choose a Bachelors course in fundamental sciences but just for the sake of this example, I’m pretty sure you wouldn’t be too happy about not receiving what you signed up for!
Now, coming back to the point, consider IIT(ISM) at a time when it was still ISM, people would toil for two years and clear the JEE Advanced, considered one of the toughest engineering entrances in the planet and invariably associated with the prestigious IITs and then make it to ISM, one of the participating institutes of the JEE Advanced. It is common sense that everyone who clears the JEE dreams of an IIT degree, after all he/she is as intelligent and as capable as a prospective IITian because he/she cleared the JEE, no one would actually want a degree from ISM even though it is a highly acclaimed and world famous institution. Thus, many people blindly chose newer IITs instead of ISM.