Humble Indie Bundle 12 is the lowest selling Humble Indie Bundle since Humble Indie Bundle 2

I think there's two issues:

First, a lot of these are really popular games, so people have them already.

Second, while $10 isn't unreasonable for a new, well-reviewed game like Prison Architect, the fact that it is so popular (and is a decent deal for that particular game), combined with the fact that people probably already own most of the lower tiers, resulted in the majority of the purchases being for Prison Architect, dragging up the average. Worse, the $1 tier is comparatively bad (they're good, but not classics like Papers Please or Prison Architect), so few people bought at that tier, raising the average higher. And as the average approached $10, people had no reason to buy for less than that, pushing it higher still.

This resulted in a bundle that was a bad buy if you already had or didn't want Prison Architect, and already had or didn't want the $1 tier. And that encompasses a huge part of their audience, so... fewer sales, despite having good games.

12 actually ended up making as much as most of the Indie Bundles, they're all around $2.5 million except for V which was $5 mil. The lower number of bundles sold but normal amount of money raised is actually fantastic for Humble and the devs that participate. Sorry cheapskates, you can no longer pay $3 and get 10 games anymore. Now you actually have to support indie devs with more than pocket change if your want their games. The lower number on bundles sold also likely means that it's no longer very profitable for resellers to hoard 50 bundles and sell them later, so that's a win for everyone involved except for people who like to profit off other people's hard work and charity. The smaller amount of bundle copies of dev's games being sold after the bundle ends is better than them selling an extra 250,000 bundles to people who paid anywhere from a cent to a dollar.

I'm interested in some of the mid-tier stuff, but the fixed price jacks up the average beyond what I'm willing to pay for it. Like right now, I'd like the Blackwell stuff, but I'm not dropping $6 for just those 3 games.

I have just started messing with this stuff personally, but for me it is less about the price and more about the selection.

So far, I have been messing with it about 2 months now, in that time, I have bought 1 humble and 1 weekly bundle and I paid enough to get everything on both of them. But in both cases, they at least had something I wanted. I personally have no intention of buying something when there isn't even a single game in the whole thing I want. I pay to play, not to pad my game library like a game of pokemon (mostly).

If they were able to put some more of the name brand stuff in there too many might buy them, even if it is just old classics.

Example: How many here would be willing to buy an ID Software bundle with the old Quake Collection in it?
How many here would be willing to buy a Counter Strike bundle or a Final Fantasy bundle or an Unreal bundle?

Never forget Indie Bundle V...