That frostfire set bonus is crit damage, not crit chance, and it is actually pretty weak. It isn’t calculating out to be a very compelling bonus for the most part. I’m not surprised it isn’t being chosen. Crit damage can be an ok stat when your crit rate gets higher, but until then, it is actually a pretty weak stat.
The crit meta is chosen by BiS when all raid buffs/debuffs and consumables are available, along with all possible gear. It doesn’t quite edge out the int meta when more restrictive options are available. The difference in output between those two meta gems is very, very small. There is no way you’d ever be able to actually notice a difference. You need more crit rate for it to win out.
I don’t understand the question about the shoulder enchants. I see it picking Inscription of High Discipline. We still have TBC enchants in the list because some people wanted to use them, but it picks the WotLK enchants when I look at it.