There is less demand for ST items and because of that, such items are more difficult to sell. This is especially the case with shitty knives such as Stained, Urban masked etc where the demand is practically non-existent. For this reason, people try to avoid such items because it's a waste of their time to try to sell it.

However, ST versions of gun skins aren't that hard to sell, for example ST Vulcans are pretty easy to sell.

So in short, until you've reached the level of trading where you can gauge how hard each ST knife is, it's a good idea to just avoid them