The 2 main options are the Swiss state insurance (LAMal) or a private frontalier insurance. A third option is the CMU which gets you onto the French state system but from what I've read is quite limited in how much you can claim (and this is not the same as the French "sécurité sociale" which you cannot subscribe to).
As far as I know the LAMal is the most expensive and similar to what Swiss residents have to subscribe to except that frontaliers are unable to reduce their premiums by opting for a higher excess (or any excess at all by the sounds of it).
So that leaves private frontalier insurance which seems to be the most flexible and apparently a lot cheaper than the LAMal. I think that is the reason but I'm no expert on this.
More info here:
http://www.travailler-en-suisse.ch/a...es_utiles.html