These are just theories that I have no hard data researched to back up, but:
1) It seems like Marvel definitely distanced itself from PHCs toward the end of Marvel Now. I could be wrong but I believe all of the Secret Wars mini series all were TPB only.
2) Premiere HCs may not have sold all that well during Marvel Now. Pretty much all of the no-dust jacket premiere HCs from that era hit the remainder pile. All of the Season Ones, nearly all of the volumes of Savage Wolverine, Gillen's Iron Man, Waid's Indestructible Hulk and Remender's Captain America have hit the clearance racks. I also believe the All-New/Uncanny Bendis X-Men and Hickman Avengers PHC books were remaindered, or at least the earlier volumes were. Nova only survived one hardcover before it was clearanced and the rest of the series went TPB only. I would assume Aaron's Thor sold well enough since they OHC'd it and it seems to be the only series continuing in PHC for ANAD, but it's possible those Marvel Now PHCs were put on clearance.
3) I think Marvel is finding that instead of hoping people double or triple dip with PHC/TPB/OHC, they can make as much or more money when they start in TPB then hit collectors with Oversized or Omnibus editions for things that sell real well, like we saw with Hawkeye, Deadpool, Ms. Marvel, the new volume of Daredevil etc. Plus with no premiere hardcovers, that means Marvel can start jacking up the price of the basic TPBs, which are now all in the $16-$20 range for 6 or less issues. And then they can eventually charge more for collectors who really want that hardcover edition because there's no step down between OHC and pure TPB. It seems like the going rate for the OHCs is $35 but I'm sure that will change soon too.
3) TPBs weigh less than HCs. The difference might not seem like much, but it definitely adds up when you're talking about shipping hundreds if not thousands of copies from the printer in China. Especially considering Marvel is still clearancing out those Marvel Now PHCs (and I would assume take a loss on them financially) after they seemingly adjusted their printing practices to try and reduce over-printing (at least as far as Omnis and OHCs go). So the shipping savings might be pretty worthwhile to basically abandon the PHC format.
4) Digital sales are supposedly increasing. I think most of the Marvel Now Era PHCs came with a digital code for the trade. Perhaps Marvel has realized the most profitable path is to give physical readers the simple TPB, have digital TPBs available for the digital crowd, and anyone who wants to double dip can buy both rather than get the windfall of the digital bonus copy found inside the PHCs. And of course if it's something to pander to the collector crowd, give them an OHC eventually.
Secret Wars and the abandonment of the PHC format for ANAD makes a great jumping off point for me as a trade collector. I'm really not interested in acquiring a bunch of cheaply made but still expensive TPBs, and I'd rather just go digital than buy these TPBs. If something good comes out in OHC, then I'll definitely be interested. Maybe way down the road I would be interested in Large TPB/Complete Collection books if they're never going to do OHCs, but even that's iffy for me personally. DC is doing me the same favor with how Convergence/DC You mirrors Secret Wars/ANAD Marvel, both in terms of continuity resets/refreshes and in terms of trade collection. Though DC is keeping most Batman books as HCs, so they still get me there.
Did anyone notice the CAPTAIN MARVEL: EARTH'S MIGHTIEST HERO VOL. 1 TPB? It's kind of interesting they're putting out a TPB collection of this series (combining the first two TPBs into one) instead of OHCs or an Omnibus. My guess would be a three volume TPB re-collection: 1 - In Pursuit of Flight/Down; 2 - Avengers: The Enemy Within (maybe w/ the Infinity issues)/Higher, Further, Faster, More; 3 - Stay Fly/Alis Volat Propriis/Captain Marvel & the Carol Corps.
It probably makes more sense to combine the series together under one banner since it has the two volume split with the Avengers tangent and Secret Wars Carol Coprs tangent in there. That way new readers can just pick up the "Earth's Mightiest Hero" volumes and not have to know to go seek out an Avengers TPB or get confused with the needless renumbering that carried over into the trades. I'm sure bookstores would also be more inclined to order these types of TPBs than order reprints of those 7 volumes, which is probably a harder sell. I wonder if strong sales on the TPB reprint collections might lead to an OHC or Omnibus maybe in late 2017 or 2018 prior to the March 2019 film.