Because you aren't getting western players to run the same bosses for 7 years.

When they add new stuff, many, if not most of us prefer to not run the two things we just spent months running and getting bored of.

Especially when there's only so much gear variation that can exist with the current system.

The only game that ran horizontal progression for more than a couple years was FFXI. EQ2 did it a couple times, not increasing level cap. We stayed at 70 for two, and 90 for 3. Level cap went to 92 in a raid patch. 95 in the next expansion. We still didn't run the old stuff often because we were sick of it by then.

Especially with the epic weapon quests and then Epic Repucussions quest to extract the clicky buff from it. Holy hell.