Come now, theyre not entirely different, at least when the first episode started and people pretty much wanted the simpsons but in space. I mean homer and bender have similar loud in your face outrageous personalities and even have a similar look with the bowl cut and the overbite thing. Obviously as the series went on people saw the differences more starkly. Plus, like homer, bender was on ALL the merch and clearly intended to be the fave.
Bart and fry, I suppose people saw them both as delinquents that never get anything right. Again as the series went on the differences became more obvious. But thats definitely what people were saying at the time