Why are there so Few 7.1 releases?

[EricBC]

I've had 7.1 for a good while and it seems like very few movies have it, and some of the ones that do are surprising, and it is really surprising that some do not, they have little else going for the movies other then sound, explosions, etc.

Dark City is amazing and the only 7.1 release I've found that was really impressive, and a good movie, most of the other 7.1 BDs I see are meh movies.

I haven't gone and looked super hard, but Avatar no 7.1? Transformers? Or am I just not in the know on this stuff?