What about people collecting the new Valiant???
And you could say the same about many comics.
What about stuff like "Bone"??? Very limited scope, as it's just one (or two?) self-contained series. But it still gets money and accolades.
Just depends on rarity, quality, etc.
Same with the Turtles. They died off, now they're back. Even if they weren't, you'd still get collectors.
You can't compare Harbinger to the Turtles or even Bone.
The Turtles have been in movies, cartoons, comics, toys, pretty much nonstop for nearly 30 years. The are constantly creating new fans, young fans.
Bone, while not as popular as the Turtles, has been reprinted countless times by a very large non-comic publisher -- Scholastic. There are a ridiculous number of copies of those in print, numbers that dwarf Harbinger's numbers. Also, the first printing of Bone #1 was much, much smaller than Harbinger, and is legitimately much harder to find.
Harbinger was a short-lived series that at one time was incredibly popular. The first issue may be a top-20 modern book now, but I don't think it's going to last. The most recent sales through Diamond were under 10,000 copies, the 199th most ordered book that month.