It's because the stuff is so damn hard to find where I live, and when I've found online suppliers in New Zealand it has been really expensive.
Once, I found a small selection of plain, block coloured organic cotton. At Spotlight of all places. It was lovely to sew with and is lovely to wear. It seems to need less ironing than the normal cotton, and has that softly crumpled linen look and feel. But I haven't found it the last few visits.
Come, NZ fabric retailers - surely you can do better than this.