I think the reason was that people thought the movie should have been based on what they think Transformers "is", and for those fans Transformers is the G1 toys/cartoon.
And that's also true for a big chunk of the audience for the movie who wouldn't know what a maximal, vehicon, a mini-con or a cyber planet key was, but would go see the movie based on memories of G1.
The movie used lots of elements from G1 but very superficially in most cases. If the movie had used new character names those complaints that Ratchet was green or Barricade wasn't a purple race car (a huge stretch that one but people make it) would have gone away. Instead people would have complained that there were new characters it the movie instead of the characters they "knew".
