I honestly believe their lack of press did it. The band people constantly bring up is Alice in Chains, who has sold over 500 000 copies of Black Gives Way To Blue. There's a few obvious reasons for this:
1) The album has had three songs in the Billboard Top 10 Rock Songs. Check My Brain and Your Decision were both huge rock hits. Comparatively, STP has Between the Lines, which was bound to hit #1 because of the anticipation. Check My Brain wasn't BGWTB's first single, it didn't make it on hype alone.
2) The amount of interviews the band has done with various groups all over the world. You can't hit up Blabbermouth for one day without reading, watching, or listening to a new Alice in Chains interview. Comparatively, searching youtube I could probably come up with 2 or 3 STP interviews from the last year.
3) Alice In Chains has a successful formula that they have stuck to through thick and thin. They don't have a pop album, a heavy album, or an art-rock album. They are what they are, and they're happy with that. People who buy the new album know exactly what they're getting. STP does not have two albums that sound alike. Core made them who they are, and despite our own personal preferences, the general public wants that sound. Its what made STP relevant and popular.
4) The enigma created by the death of Layne Staley and the 10 years of relative inactivity. STP doesn't really have this, and to be honest, Scott Weiland's PR image is brutal, with immovable rock giants like Slash constantly calling him out in the press.
If they did some more interviews and made an album that sounded like Core that generated a few serious hits, STP would have sold 500 000 copies. Sadly their management seems to be non existent. I'm glad we have an STP album, and I really like it. The only reason I'd be dissapointed in the numbers is because the band might look at putting out new music in the future as being a bit useless. They seem to be turning into a club band...Alice In Chains just headlined MSG with their new tour...