I think that the "what changed" was definitely Sormo's re-awakening. No doubt about that. There will probably be more theories in a couple of weeks when we get to that chapter. I will bring it up again then. Hopefully we can get some great input at that time. Shalter and Saltman Z are both usually really great at explaining things to me when I'm confused on something.
Daydreamer, first of all, welcome to the re-read. Glad you decided to join us.
“We're told that Obelisk has been inactive in 7 Cities but are we ever told why and what changed that?”
First for the “WHAT CHANGED?” In Chapter 10, after Duiker catches back up to Coltaine, List explains to Duiker about Sormo. Corporal List said, “It’s said he’s awakened the spirits of the land and the air. From a sleep of centuries…”
Now for the “WHY?” You may recall that in Chapter 10, Sormo takes Duiker (and some others) into an alternate “place/time” where everything has been destroyed by ice and they are standing at the foot of a giant hill of ice full of dead (and undead) beings, machines, buildings, etc. Duiker specifically notes dead, K’Chain, Forkural Assail, Imass, First Empire, First Human Empire, etc. all in the ice. Sormo explains to him that Jaghut raised the ice, sometimes slowly and sometimes very quickly, depending on their need. It appears that the ancient Jaghut here in Seven Cities who raised this particular ice, did so very quickly, so quickly, that nothing escaped. Everything was caught and destroyed in the ice. Not even Ascendants could escape the ice, as Sormo’s reason for being there is to destroy the Semk god by feeding it to the reawakened earth spirits or spirits of the land. As the Semk god emerges from the ice, the newly awakened earth spirits swarm in and tear him to pieces. Sormo explains to Duiker that the highest Omtose Phellack rituals (such as this one) never die. “They rise, subside, and rise yet again.” So I read that to mean that Burn/Obelisk (goddess of the earth) was denied access to Seven Cities because of the Jaghut’s unleashing of Omtose in ancient Seven Cities, an unleashing that is still alive today.