Originally, mostly for ideological reasons and because people simply followed the anti-Microsoft trend. Technologically, IE was not a bad browser when it was released and had quite a few advantages over Navigator/Communicator.
Of course, when Microsoft decided to abandon the whole thing and the competition (i.e. Mozilla) continued working on it, IE slowly lost these advantages.
So, down the line there were certainly also technological reasons why people were not too fond of it, but most of the dislike originated from said anti-Microsoft ideology. Of course, we are speaking here about 20ish years ago.
Of course, when Microsoft decided to abandon the whole thing and the competition (i.e. Mozilla) continued working on it, IE slowly lost these advantages.
So, down the line there were certainly also technological reasons why people were not too fond of it, but most of the dislike originated from said anti-Microsoft ideology. Of course, we are speaking here about 20ish years ago.