If I could speak for Jkn a little, I think he would agree with you on this. The gist is that there aren't lyrical vocal pieces in ambient, well, almost never, and the ambient that does have vocals like the ones you mentioned doesn't make ambient any more popular.
Although some vocal ambient might have a decent amount of crossover appeal, it's not going to be in the stratosphere of sales (only in our minds
) This probably has to do with facts like it's still ambient in nature, and the lyrics if there are any, are just not as relatable. Take for example the group Primeaux and Mike, a Native american duo who uses ambient effects along with their very harmonious chants. It's very nice, and surely it attracted people to the 'scene' who never heard of Oophoi or Celer, but obviously it's still a niche- style that isn't going to burst any charts...
Seems like the only times ambient does get popular is when a well known artist goes in that direction, or the music is also a soundtrack to a very popular movie. Trent Reznor's "Ghosts", and I also remember seeing people with the Trainspotting soundtrack who probably got turned on to Brian Eno but otherwise had little or no ambient in their collection.
I wonder what segment of the population has the most disposable income? Young people? I know that's when I was able to buy more music, when I had less financial responsibilities to worry about. And what percentage of ambient listeners are young? Probably much, much less than pop and rock music. It's a contributing factor, if not THE major one.