I keep going back to the question, “Why did knitting belts survive when knitting sheaths died out?”
There are three basic answers that keep coming to me.
The first is that there was more social stigma to using a knitting sheath than to using a knitting belt. As the Fair Isle knitting industry started up in the late Victorian era, the knitters needed to knit as fast as possible. Previously, most professional knitters had used the cheaper knitting sheath, thus knitting belts escaped the social stigma attached to professional knitting. However, for Fair Isle, a knitting belt was as good as a knitting sheath without the social stigma. The knitting belt was more akin to the knitting heart worn by ladies for leisure knitting and thereby socially acceptable.
Second, Fair Isle sweaters started being knit for tourists or for export in late Victorian times. They were knit more loosely than the older sweaters knit for local consumption. The firm support of a wooden knitting sheath was not required for this looser knitting. A knitting belt was a softer, more comfortable device that gave adequate support to the needles for the type of knitting being done.
Finally, even Shetland fishermen were using “steam” vessels by 1900, and did not have the same need for warmth that earlier fishermen on sail boats required. (Working in the rigging in foul weather puts special requirements on the seaman’s clothing.) Thus, in late Victorian times the need for the very warm clothing that could only be produced by fine steel needles was much reduced. (If your boat has an engine, and you do not need to go into the rigging, then a “pea coat” is almost as good as a gansey, and much cheaper to produce. Fishermen are nothing if not thrifty!) Therefore, the need for knitting sheaths to support fine steel needles died out. Over time the knowledge of how to make and use knitting sheaths died out, while knitting belts continued to produce Fair Isle knitting for tourists and export.
If you are going to be knitting Fair Isle sweaters, a knitting belt really is a good tool. It is better than “pit knitting” or trying to hold a #2 DPN in your thigh crease.
On the other hand, if you want to knit fine socks, warm ganseys, or an Aran for your mouse, then a knitting sheath is better. The down side of knitting sheaths is that they must fit the needles. The upside of knitting sheaths is they are the most ergonomic, and you can knit things with them that are very difficult to knit in any other way.