as a 6 string user here..
i get it. i can only by sets including low b and high c.
luckily, there really is no shortage of strings. i have lots of options, most are pretty damn good.
i wish ever single variety of string were available in a 6 string set. but, i'm understanding that this is not realistic.
i 100% understand that companies have to use sales as a guide for what they can keep in stock. but, i don't think creating another diameter of the same string is that much of a big job that it would really cost them money? The problem would likely be that they cant charge significantly more for those extras.
if they offered this, and lets say adding the C string cost you $8 more rather than the $2 or $4 or whatever you'd expect, then you would not be willing to spend, and that's where the problem comes from.
have the companies in question tested these waters: would the "guy who needs the B, or C, or both" be willing to pay the appropriate markup to make this mutually beneficial? I really don't know. it's not a huge part of the market and if you consider all the factors its quite remarkable that there are so many choices for 6 string players these days.