There wasn't a lot of promotion for it in mainstream methods because it was released in an era where heavy rock wasn't an in thing anymore. This wasn't the 2000s anymore. Releasing GATS as a lead single was a risky move, and none of the five singles released charted in the Hot 100 nor the UK charts (UIG managed to make it on the year-end rock charts though). In a way, just like OML was focused on personal story interviews, THP focused on the rock crowds. They played through festival after festival. They performed in a lot of rock-radio sponsored shows. The only mainstream appearances were the iHeart Radio release party and Jimmy Kimmel Live. They made the unconventional move of releasing five official singles ahead of the album, all in unconventional fashion. I think they made a mistake with GATS, where they released it through Shazam and they made probably their worst video ever with it being a tie-up with Project Spark. "Wastelands" was leaked on the net before officially released. Also, it didn't help that they only played half the album on their tour, and most of their early 2015 tour was cancelled because of yet another Chester injury. But overall, looking back, it was never going to be a massive success since it was against the indie/pop/R&B/electronic trends of that year. It did get good reviews and some fans back (temporarily until OML is released lmao) and the band is proud of it, but its genre (plus a few bad choices imo) prevented it from having the success that LT had. You can argue that THP had more promotion that LT, but not mainstream-wise promotion.
Well, for me THP was a really good album, consistent with good lyrics but if they didn't promote it well or well enough they obviously corrected their mistakes this time because OML is all over. GATS is a really good song but if you don't have a good video and your other song leaks before, then they did someting wrong or it was just a series of misfortunes because they also didn't have time to promote it if Chester got injured (damn, that man gets injuries all the time). I just wish more songs will be played on this tour.