might be because no one is casting for the 6's. the 22 and 7mm's do any thing you'd expect from the 6's. get a BUNCH of people asking for the 6mm mold and the bigger companies will jump on the wagon for a piece of the $$. other wise they won't invest time and money on what THEY see as a losing proposition! HOW many .24/6mm calibers are around? .243, 6mm & 240 Weatherby, none of these are considered as cast bullet choices. The smaller wildcat 6's are fed jacketed far more than they ever would be cast.
I use cast in the 7TCU and get looked at as if I had eight eyes by most of the shooters at the range. Every one there KNOWS cast bullets are NOT accurate! After they shoot a few of mine ,though, they start to get interested. I have lent my molds and sizers to a couple shooters. They cast a hundred or so bullets and size/load them. They quickly return my equipment saying casting is TOO much work!