Didn't receive return offer and feedback wasn't clear

I interned over summer in a top tier BB in S&T sales. I honestly thought I was doing well and was enjoying it. They had quite a few interns as well. I always made sure I was asking for feedback and never had anything negative, so naturally expected a return offer.

At the end, they only took 25% of the interns! We were all quite shocked, trying to get in touch with people as to what had happened and not getting much of a response, most never replied to emails/calls. My feedback from HR was that 'I didn't seem dedicated to sales' and that maybe I'd be better at research or trading. I asked how to pursue that, and was just told to apply when jobs are available on the careers site. One of my managers said that I could have 'maybe been a bit more assertive' and that the desks were filling out with grads. They didn't go back to recruit for graduates that round so all I can assume is that it was a headcount thing? I am not full of myself, hence why I was constantly asking for feedback. I feel to not give me an offer, something significant must have happened along the way and yet no one said anything, and I have no idea how to improve or what to change. I ended up getting to multiple final round interviews for other top and mid tier BBs but got no offers, and now I'm a little stressed. They all of course asked why I wasn't going back to firm XX and I had to make up some bull side story.

I just don't understand how such a successful BB could not have the capacity to take more than 25% of interns in ~ok~ economic times. What would you expect is the reason behind this? Everyone makes out that if you don't make any huge mistakes then you're set for a return offer, but that was not our experience at all. Would you usually know what you'd done wrong if you didn't get a return offer? Thanks for responses :)