The main reason is that no major manufacturer currently sells a good quality 700c coaster wheel or complete bike. Also, no real improvements have been made in coaster brake technology in basically ever.

I think they will become more popular than they currently are, especially when the trendy fixers move on to something else and get rid of or coasterize their bikes.

A 700c coaster makes more sense for me, but they aren't any better or worse than fixed. It is totally a matter of personal preference.