Here are some points to consider if you're thinking why IIT Madras is not popular like IIT Bombay/Delhi, though Chennai is a metropolitan city:
Overall, while Chennai has some concerns, it is by no means an inferior option compared to other cities, and IIT Madras could be an excellent choice for you.
IIT Bombay and Delhi are more popular than IIT Madras because these colleges are located in Metropolitan cities. Also, in terms of publicity, IITB has done a great job. While IITM is quite laid back about advertisement, publicity, and letting people know of the happenings inside the campus.
Also, the weather of Chennai is quite hot and humid which might be tough to get used to for some people. The language can also create a barrier since the large population of Chennai is Tamilian. But you can make do with English.
Other than these factors there are no such differences between IITM, IITB and IITD. In fact, there is no such major difference between the top 5 IIT, if we judge them based on infrastructure, alumni, quality of students, placement, extracurricular activities, entrepreneurship, etc.
Overall, IIT Madras is a reputed institute and a great option for your studies. So, don’t base your decision on the minor issues mentioned above.