Because it isn't that good a film. It's okay. I enjoyed it, but it's a triumph of technological wizardry over plot and characterisation. That seems to be ever the case when a film receives the sort of hype that Avatar got before release. Think of the first Harry Potter film, or Jurassic Park. Both had huge pre-release publicity and both did a storming trade at the box-office. How many of us sat through them, however, and thought, 'Yes, not bad, but it isn't that good either?'
One day they might produce a movie that is well-written, well-acted, has an interesting storyline and believable characters AND gob-smacking special effects. That would be worth seeing.