I think there are a variety of reasons. Viceroy cards makes a good point, some collectors have changed their spending habits to less costly sketches since they can get less established artists who do high quality work at a lower cost than in the past and get more bang for their buck; I think with the overall increase in quality of sketch cards in many products, sellers are also willing to let the top sketches go at a cheaper price, because it's easier to recoup the cost of boxes/cases with the increased amount of $30-50 sketches you now pull.
I think oversaturation is also a problem. In the next month or two after some more sets drop, in a 13-14 month period, we will have had:
3 Rittenhouse Marvel sets
3 Cryptozoic DC sets
4 Upper Deck Marvel sets
And these are just the major comic book based releases! The only lucky thing is that with UD, Marvel Premier was a high end set that attracted mostly a different set of buyers from a lot of other sets, and the other UD sets didn't rely only on sketches (and so you got a sketch in every other box instead of in every box), otherwise we would have had even more saturation.
On the artists you mentioned specifically, I think the Glebes have been hurt by the amount of sketches they have put out in the past 6 months to a year. Being an incentive for the past 2 Rittenhouse sets is a big honor, but it means that a LOT of their sketches have hit the market, especially with how the handful of big breakers break Rittenhouse products.
On Charles Hall, I think he really hurt the value of his sketches when he started doing more sketches, but of lower quality. That happened in both the Batman and Marvel Bronze Age sets, and it dragged the prices of all of his sketches down. I think too many people look at past ebay sales to determine value, and suddenly instead of seeing $150-200 sales, you are seeing a number of $50-75 sales, and people are having trouble justifying the higher prices even for the better quality sketches.
I think to some extent that has happened to Katie Cook as well, since some of her recent sketch cards have been more like headshots with little or no background. I also feel that Katie Cook's prices got pushed up so high by a handful of big collectors who fought over her cards, and now they don't seem to be going after her sketches quiet as hard.