Because there is a century now of diamonds being associated with certain cultural elements in US life, and that's not easy to take away overnight. Lots of people expect a diamond ring as part of an engagement - not just the future bride, but their friends, family, co-workers. A sapphire ring or an opal ring or a ruby ring will not be easily accepted - it will be seen as weird, or cheap, or anti-traditional, etc.

Now sure, this concept was manufactured to a great extent through marketing, and it can be replaced or just fall out of favor. But established culture changes very slowly, and there's no "other gemstones cartel" to throw money at this the way DeBeers did to establish the diamond engagement ring in the first place.