What are the reasons that IIIT-Hyderabad ECE is not as good as its CSE?

The question is highly illogical as no branches are comparable, rather it’s a matter of preferences. You can say that placements of CSE are better than ECE but to say that to the entire branch would be making a false claim. 

In any engineering institute you will find the average packages of the CSE branch higher than other branches but does this mean the other branches are not as good? The answer is no. The placements are directly affected by the demand in industry and it has nothing to do with the credibility of the branch. If you are genuinely interested in ECE, then CSE won’t be a good choice for you irrespective of how lucrative the package you are offered. So it’s a subjective matter. 

People tend to adopt the sheep mentality and flock to CSE just because everyone is doing so and fail to analyze their own passions and interests. The increasing size of packages and international offers has resulted in a preconceived notion that CSE is better. In reality, both ECE and CSE branches are equally good with equally good opportunities, faculties, and education. 

So it’s advisable to make the decision without any preconceived notions.