I don't think it has anything to do with impact strength or durability or whatever. There are plenty of carbon mtb frames on the market. It probably has to do with the fact that cx racing is still a niche market, altho it is gaining popularity in the US. Manufacturers aren't going to put a lot of time, money, and R+D into making multiple models of high-end cross bikes if people aren't going to buy them. Most people buying 'cross bikes are already mtb-ers or road racers or just casual riders looking to try something new. It's unlikely they'll buy a $3000+ carbon cross bike as their first cross bike just to see if they like racing cross.
That being said, given the increase in popularity of cross racing, gravel grinder events, etc, there have been a lot more carbon cross bikes coming out in the past few years.