My wife has a diamond engagement ring and wedding band. Since then I've bought her several cubic zirconias. I can tell the difference (I'm a geologist by training) because the CZ's have too much fire, but I'm not sure I would ever see the difference if I didn't know in advance. And moissanites are much closer even than CZ in diamond-like properties. CZ, however, is way cheaper. Like $10 for a 1-carat stone.
Could be that moissanite is just expensive enough to qualify as an engagement stone (where CS is way too cheap).
Additionally, I think CZ has negative connotations that moissanite hasn't acquired yet. There's been longer for sitcoms to make jokes like "the jerk got me a cheap cubic zirconium!", which isn't the case for moissanite.
Could be that moissanite is just expensive enough to qualify as an engagement stone (where CS is way too cheap).