Probably because women of power buy their own pearls and appreciate pearls offer more visual impact for their money. Whereas starlets tend to have their diamonds lent to them by jewellers for publicity purposes.
My 2p
DK
Pearls also aren't as good for the red carpet because, as we know, they can be challenging to photograph. Unless starlets love pearls, they have little incentive to wear real ones-- It's hard to be sure at that distance. Politicians are more likely to be speaking to others individually for part of their day.
I think younger women will decide they like pearls if they get exposed to them. But unless I'm very wrong about my peers, PR work will have to be done to replace the boomers' level of pearl consumption in the under 30-crowd.
Life is pretty casual for most young people, so getting people to pair pearls with attire other than a black cocktail dress is important.
For many young people, the fact that a living thing died in the production of the jewel is going to be hard to surmount. Emphasizing (when true) that pearling is a source of food and jobs/income for coastal people may help.
Michelle may help and hurt pearls. She has people talking about them, which is great. I have heard she may wear fakes for ethical reasons and to avoid bad press, but fakes aren't a substitute for the real thing visually or in terms of enduring value. The political inclinations of many who unabashedly eat meat and wear dead animals may cause them to avoid anything Obamaish. It'll be interesting to see what happens, but any press is probably good press.