I'm going to speculate here.
This is because of Infinity War. Now I know that's a pretty canned answer all around (it's Marvel's fault!!!!1111!!) but hear me out.
HT is likely interjecting Infinity War product production between runs of their previously preorderable figures. Someone said that patrons of the Toy Sapiens show on the 20th will be able to already purchase Black Widow and place a deposit on Cap. They want these out faster to capitalize on the hype, since they'll need to do it all again for Avengers 4 for the same characters.
While yes, IW products are figures with a lot of reuse, they still need to be manufactured and there's only so much manpower HT can employ at once. So what do they do? They run off 200, 300 copies of Jack, Joker, Spidey or Anakin and distribute them; then they return to IW production. In the meantime, people see final product pics of Jack, for example, and scramble to get one. He waitlisted days after the first pics came out. Same thing with Joker. Same thing with Anakin. Same thing with Spidey.
Isn't it a bit odd that all of these would follow the same template that ultimately leads to no leftover stock? It's capitalizing on supply, demand, and artificially (but conveniently) driven hype. Two birds, one stone. This gets people paying on and reserving figures before the onslaught of Marvel releases that is coming ("Run from it..."), which frees up funds when new figures hit again.
Not to diminish the quality of the listed figures - they all are worthy of the attention they're getting, but you can't help but say that rate of sale is suspect - when was the last time normal release figures waitlisted before release? And suddenly four of their figures do it in a two month period shortly after a "limited batch" showed up?
So while I like to let it be known I'm less than thrilled with SS's shipping practices and that it's driving me away from them, I don't necessarily think they are to be blamed on this one.